Welcome back to Rewind Monday, where we take a closer look at the weekend's top storylines in college football. This week, we check in on the last remaining undefeated teams—how they got here, why they’re here and what’s in their future.

Clemson, Oklahoma, SMU, Alabama, Memphis, Ohio State and Baylor. What do they all have in common? They’re undefeated. All in all, there are 18 undefeated teams in college football heading into Week 6 of the season. That’s a lot. Some of them are lucky to be here, while others have passed their four or five tests with flying colors.

This weekend, many of them will face their toughest opponents. One game, in fact, pits undefeated teams: Auburn at Florida. Iowa goes to Michigan, and Baylor travels to Kansas State. Memphis is at UL-Monroe, and Boise goes to UNLV. Speaking of Las Vegas, the odds-makers tell us at least three undefeated teams are supposed to lose this weekend.

As we move into Week 6 of the season, here’s an analysis of the remaining undefeated teams, listed in order of who we believe has the best chance to finish the regular season undefeated.

Clemson

Record: 5–0

Rank: No. 2

Best win: 24–10 vs. Texas A&M, Game 2

How they’re undefeated: The Tigers escaped Chapel Hill with a 21–20 win after stuffing North Carolina’s potential winning two-point conversion attempt. Clemson showed some vulnerability.

Toughest test left: at South Carolina, Nov. 30

Ranked teams remaining: 1

A case for finishing undefeated: Wake Forest is Clemson’s only remaining regular season opponent that is ranked, and the others aren’t really close to being ranked. In fact, the Tigers’ remaining schedule ranks 58, the second-easiest road of any top-15 team in ESPN’s power index rankings.

A case for not finishing undefeated: You’re going to need a good lawyer to argue this one, even after the debacle at UNC. Clemson’s toughest remaining regular season opponent is a 2–3 South Carolina team that lost at Missouri. Clemson might not face its first top 15-caliber team until the College Football Playoff (ACC championship game included). In short, we have no case except the outing against the Tar Heels.

Oklahoma

Record: 4–0

Rank: No. 6

Best win: 55–16 vs. Texas Tech, Game 4

How they’re undefeated: The Sooners and their high-flying offense are scorching defenses. QB Jalen Hurts and Co. have scored at least 48 points in each of their first four games.

Toughest test left: vs. Texas, Oct. 12 (Dallas)

Ranked teams remaining: 1

A case for finishing undefeated: The Sooners should be favored in each remaining game, and only one of those games is against a team ranked in the current top 25: OU vs. Texas in Dallas. The Longhorns have proven so far this year that their defense can be exploited (see LSU and Oklahoma State).

A case for not finishing undefeated: A Jalen Hurts injury? We’re kidding, of course. The Sooners not only must get through the top-15 Longhorns, but they also have to travel to Kansas State, Baylor and Oklahoma State, all potential tricky road contests.

Steven Branscombe/Getty Images

Ohio State

Record: 5–0

Rank: No. 4

Best win: 48–7 at Nebraska

How they’re undefeated: There is only one team ranked in the top 10 in both total offense and total defense. The Buckeyes are eighth in offense and second in defense. OSU’s defense (223.8 yards a game allowed) might be more impressive than the Justin Fields-led offense—and that’s saying a lot.

Toughest test left: vs. Wisconsin, Oct. 26

Ranked teams remaining: 4

A case for finishing undefeated: Three of the four ranked opponents remaining on Ohio State’s schedule play at The Shoe. The fourth, Michigan, has shown us its problems. The season-ending game against the Wolverines was supposed to be OSU’s stumbling block, but that doesn’t look like the case any longer.

A case for not finishing undefeated: The Buckeyes have one of the toughest remaining stretches of any undefeated team, but also keep in mind that their early-season schedule was light. After all, their best win is a team that lost at Colorado.

Georgia

Record: 4–0

Rank: No. 3

Best win: 23–17 vs. Notre Dame

How they’re undefeated: Mostly because of a defense that’s ranked sixth nationally in scoring. The Bulldogs are ruthless on that side of the ball, and they carried the offense in the victory over the Irish in a top-10 clash in Athens.

Toughest test left: at Auburn, Nov. 16

Ranked teams remaining: 3

A case for finishing undefeated: Georgia will be pretty heavy favorites in all of its remaining games, except the trip to Auburn. The Bulldogs have a competent, veteran leader in Jake Fromm, a bruising running back and, again, that nasty D.

A case for not finishing undefeated: Georgia has lost two of the last three times it has visited Auburn including a 40–17 whipping in 2017. This won’t be an easy game. Neither will the duel in Jacksonville against Florida. UGA hosts Missouri and Texas A&M in its final two homes game, neither necessarily pushovers.

Boise State

Record: 4–0

Rank: No. 16

Best win: 36–31 at Florida State, Game 1

How they’re undefeated: The Broncos stormed back from an 18-point deficit in Tallahassee with a true freshman quarterback calling the shots. They survived games against Marshall, 14–7, and Air Force, 30–19.

Toughest test left: at BYU, Oct. 19

Ranked teams remaining: 0

A case for finishing undefeated: Boise will be favored in the rest of its games, and the Broncos have made a habit out of dominating the Mountain West. They’ve finished first in their division in five of the last seven seasons.

A case for not finishing undefeated: The Mountain West is in the midst of one of its best seasons in league history. The league took down eight Power 5 programs in the first four weeks, and five of Boise’s eight remaining games are on the road. Also keep in mind, Hank Bachmeier is still a true freshman QB.

Alabama

Record: 5–0

Rank: No. 1

Best win: 42–3 vs. Duke, Game 1

How they’re undefeated: You thought Clemson’s first month was a cakewalk—look at Alabama. Its best win is over a basketball school. But no matter the competition, the Crimson Tide appears to be unstoppable again this season.

Toughest test left: at Auburn, Nov. 30

Ranked teams remaining: 3

A case for finishing undefeated: QB Tua Tagovailoa and his quatro of freakish receivers can score consistently on anyone (well, except for Clemson last season). But Clemson isn’t on the regular season schedule and neither is Florida nor Georgia.

A case for not finishing undefeated: Two of the last three Iron Bowls played in Auburn have gone the Tigers’ way. The Tide has to visit some other hostile, loud places too (Texas A&M and Mississippi State), and its defense has shown some holes.

Wisconsin

Record: 4–0

Rank: No. 8

Best win: 35–14 vs. Michigan

How they’re undefeated: Have you seen the Badgers’ defense? The unit opened the season pitching a scoreless first 10 quarters. And then there’s that whole thing of having Jonathan Taylor as your running back.

Toughest test left: at Ohio State, Oct. 26

Ranked teams remaining: 3

A case for finishing undefeated: The Badgers win the old-fashioned way. They’ve got a bruising running game (20 nationally) and a crushing defense (first nationally). They can play keep-away with the best of them, and that’ll be important against fiery offenses like Ohio State.

A case for not finishing undefeated: Surely there will be a game this season—maybe against Michigan State, Iowa, Nebraska or Ohio State—when Wisconsin can’t rush the ball at will. And in that game, the Badgers will have to find a consistent and explosive passing game, something even missing from this past Saturday’s survival against Northwestern.

Tom Pennington/Getty Images

SMU

Record: 5–0

Rank: 24

Best win: 41–38 at TCU, Game 4

How they’re undefeated: Former Texas quarterback Shane Buechele is averaging nearly 300 yards a game and completing 66% of his passes. The Mustangs open the season with a big road win at Arkansas State and whipped TCU (at one point, they led 31–17 in the second quarter).

Toughest test left: at Memphis, Nov. 2

Ranked teams remaining: 0

A case for finishing undefeated: The American Athletic Conference is one of the better Group of Five leagues, but the Mustangs don’t face a team in the current top 25 and they’re favored in all but one remaining matchup.

A case for not finishing undefeated: The road schedule is no pushover, even if it includes zero ranked teams. Houston and Memphis, in back-to-back weeks, will be a tough challenge.

LSU

Record: 4–0

Rank: No. 4

Best win so far: 45–38 at Texas, Game 2

How they’re undefeated: Two words: Joe Burrow. The senior quarterback has put up some of the nation’s most impressive numbers, and during a Big 12-like second-half shootout at Texas, Burrow out-dueled UT QB Sam Ehlinger.

Toughest test left: at Alabama, Nov. 9

Ranked teams remaining: 4

A case for finishing undefeated: LSU might have one of the better teams of any undefeated squad. This is not your grandpa’s LSU or your dad’s LSU or even your older brother’s LSU. This offense is equipped to now score on, say, Alabama, a team that’s shut out the Tigers in two of the last three years.

A case for not finishing undefeated: The Tigers have to travel to Tuscaloosa and host three toughies: Florida, Auburn and Texas A&M. Best-case scenario seems to be 11–1. There’s a worst-case scenario out there that has the Tigers defense continuing their slide and they lose a bunch of shootouts.

Penn State

Record: 4–0

Rank: No. 12

Best win: 59–0 at Maryland, Game 4

How they’re undefeated: Because Pitt attempted—and missed—a field goal on fourth-and-goal from the 1 with less than five minutes left in the game and down seven. We’re only half-joking. Pat Narduzzi’s decision certainly helped the Nittany Lions get to this point, but PSU’s second-ranked scoring defense is the bigger help.

Toughest test left: at Ohio State, Nov. 23

Ranked teams remaining: 4

A case for finishing undefeated: The Lions appear to have a competent quarterback replacement for Trace McSorley in Sean Clifford, who has nine TDs and one INT. Also, their top two rushers are averaging at least seven yards a carry.

A case for not finishing undefeated: Have you seen the schedule? They’ve got three top-25 road games: Ohio State, Michigan State and Iowa. They also host Michigan. It gets no easier from here for coach James Franklin’s squad.

Florida

Record: 5–0

Rank: No. 10

Best win: at Kentucky 29–21, Game 3

How they’re undefeated: We don’t know. Seriously, the Gators have escaped in ugly performances against Miami and at Kentucky, and they’ve lost their starting quarterback.

Toughest test left: at LSU, Oct. 12

Ranked teams remaining: 3

A case for finishing undefeated: The Gators have found something in former back QB Kyle Trask. He’s completing 77% of his passes. With at strong defense, a steady hand might be all UF needs behind center.

A case for not finishing undefeated: Aside from facing three top 10 teams—two away from home—the Gators have problems, and they’ve been exploited by the likes of Kentucky and Miami. What will LSU, Georgia and Auburn do?

Auburn

Record: 5–0

Rank: No. 7

Best win: vs. Oregon 27–21, Game 1

How they’re undefeated: It’s still hard to believe that Auburn, with a true freshman starting quarterback, beat Oregon, won at Texas A&M and thrashed Mississippi State. It says a lot about DC Kevin Steele and his unit.

Toughest test left: at LSU, Oct. 26

Ranked teams remaining: 4

A case for finishing undefeated: The Tigers have excelled with coach Gus Malzahn when they’ve had a smart quarterback, a bruising running game and a strong defense. Check, check, check so far this season.

A case for not finishing undefeated: All four of Auburn’s remaining ranked opponents are in the top 10, and two of them are on the road, at Florida this week and at LSU next week. The remaining strength of schedule is the toughest among teams in ESPN FPI’s top 10.

Matthew Holst/Getty Images

Iowa

Record: 4–0

Rank: No. 14

Best win: at Iowa State 18–17, Game 3

How they’re undefeated: Well, three of their four wins are to Middle Tennessee, Miami (Ohio) and Rutgers, so that helps. Statistically speaking, the Hawkeyes defense has allowed a combined 34 points this year.

Toughest test left: at Wisconsin, Nov. 9

Ranked teams remaining: 3

A case for finishing undefeated: QB Nate Stanley hasn’t thrown an interception yet, and Iowa has three running backs who have run for at least 200 yards. The defense ranks in the top five in total D and scoring D.

A case for not finishing undefeated: We really haven’t seen the Hawkeyes play a great team, and they’ve got at least two of them left on the schedule: Penn State and Wisconsin, not to mention a trip to the Big House this weekend.

Appalachian State

Record: 4–0

Rank: NR

Best win: at North Carolina 34–31, Game 3

How they’re undefeated: RB Darrynton Evans is carving up defenses, averaging seven yards a carry and 117 a game. The Mountaineers got a fourth-quarter pick-six against Coastal Carolina last weekend to survive a scare.

Toughest test left: at South Carolina, Nov. 9

Ranked teams remaining: 0

A case for finishing undefeated: Sure, App State has to travel to an SEC team later this year, but the Sun Belt slate isn’t so treacherous. Remember, this program posted 11–2 seasons in two of the last four seasons.

A case for not finishing undefeated: App State will be an underdog in at least two remaining games, and that starts this weekend with a game at the Ragin' Cajuns. A season-ending trip to Troy could spoil everything.

Memphis

Record: 4–0

Rank: NR

Best win: vs. Ole Miss 15–10, Game 1

How they’re undefeated: Just 10 teams rank better than Memphis in total defense, and QB Brady White is averaging 228 yards a game. UM survived SEC West foe Ole Miss to start the year and then stormed back in a win over Navy last week.

Toughest test left: at Houston, Nov. 16

Ranked teams remaining: 0

A case for finishing undefeated: The American Athletic is maybe the toughest Group of Five league, but the Tigers bypass UCF in the regular season and play some of the tougher conference teams at home (SMU, Cincinnati and Tulane).

A case for not finishing undefeated: Navy put a real scare into Memphis. The Midshipmen, 3–10 last season, led 20–14 at halftime and dominated much of the first three quarters, exploiting some flaws with the Tigers.

Wake Forest

Record: 5–0

Rank: No. 22

Best win: vs. North Carolina 24–18, Game 3

How they’re undefeated: QB Jamie Newman averages more than 350 yards a game in coach Dave Clawson’s zone-read spread offense, but it helps that the Demon Deacons have played the likes of Rice, Elon and a Boston College team that earlier lost to Kansas.

Toughest test left: at Clemson, Nov. 16

Ranked teams remaining: 1

A case for finishing undefeated: Newman has thrown for 14 touchdowns and just three interceptions. He’s not only fun to watch, but he’s capable of leading the Deacons to wins even with the defense falters.

A case for not finishing undefeated: For starters, there’s that whole you play at Clemson thing. Also, do we really know how good or bad Wake is? The Eagles took them to the brink over the weekend and Utah State did the same to open the year.

Baylor

Record: 4-0

Rank: NR

Best win: vs. Iowa State 23–21, Game 4

How they’re undefeated: Well, the Bears have only left home once, and that was a trip to Rice, where they escaped with a 21–13 victory. Kicker John Mayers had to make a 38-yard field goal over the weekend to preserve the record and beat Iowa State.

Toughest test left: vs. Oklahoma, Nov. 16

Ranked teams remaining: 2

A case for finishing undefeated: Charlie Brewer is having a heck of a first four games. He’s thrown 10 TDs, no interceptions and is nearly at the 1,000-yard mark. He’s got two rushing scores and 96 yards on the ground as well.

A case for not finishing undefeated: The Bears will likely be underdogs in five of their final eight games, including this weekend at Kansas State. In November, they get back-to-back-to-back weeks of this: at TCU, vs. Oklahoma, vs. Texas.

Minnesota

Record: 4–0

Rank: NR

Best win: at Purdue 38–31, Game 4

How they’re undefeated: They are 4 for 4 in one-score games, winning in double overtime against Fresno State, escaping against Georgia Southern and South Dakota State before surviving a comeback attempt from the Boilermakers.

Toughest test left: at Iowa, Nov. 16

Ranked teams remaining: 3

A case for finishing undefeated: The Gophers avoid league foe Ohio State, and they’ve got one of the nation’s most productive passing offenses. QB Tanner Morgan ranks 25 nationally averaging 273 yards a game.

A case for not finishing undefeated: They’ve played some poor teams really, really close over the first month. That doesn’t bode well when the big dogs come barking (ie: Penn State, Iowa and Wisconsin).