Ahead of the 2015 season, SI.com is ranking the NFL's best at every position. After ranking the league's top 10 defensive lines, offensive lines, wide receivers, running backs and quarterbacks, we turn our attention to the NFL's 10 best linebacker corps.

Because the differences between 3–4 and 4–3 linebacker corps can be so glaring and the responsibilities so varied, we split these rankings into two sets of top fives.

3–4 Linebacker Corps

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​1. Baltimore Ravens: This group is so deep, it can lose Pernell McPhee to free agency and still rank on top of the 3–4 corps. Of course, we start with Elvis Dumervil and Terrell Suggs, who combined in 2014 for 29 sacks (17 for Dumervil, 12 for Suggs) and 111 total pressures (56 for Suggs, 55 for Dumervil). While Dumervil is the guided-missile pass-rusher, Suggs is the more dimensional player who is also very effective against the run. Inside, the Ravens feature Daryl Smith and C.J. Mosley—Smith is both a tackling machine and a great asset in short-to-intermediate coverage, while Mosley made the Pro Bowl as a rookie in 2014 with 99 tackles, three sacks and two interceptions.

As for missing McPhee's multi-gap pass rush abilities, the team hopes fourth-round rookie Za'Darius Smith can take that role right away.

"[He has] the physicality, the heavy hands, the high motor. He has pass rush ability even maybe more so than we saw from Pernell coming out," Ravens head coach John Harbaugh said of Smith in May. "He has hips, and he can get around blocks. If he develops like Pernell did, we're going to have something. He's a great young man. He's a very hard worker, and he plays really hard. So, those are all characteristics that Pernell has as well."

• FARRAR: Ranking the NFL's top 10 defensive lines entering the 2015 season

2. Kansas City Chiefs: As with the Ravens, the Chiefs' linebacker quartet starts with its pass-rushers, and there are very few in the NFL better than Justin Houston. The fifth-year veteran came within half a sack of Michael Strahan's single-season record with 22 in 2014, which led to the six-year, $101 million contract he signed in July. It's the biggest contract ever given to a linebacker, and if Houston keeps on at his current pace, the compensation is appropriate. On the other side, Tamba Hali is coming off his lowest sack total (6.0) since 2008, but he's still a good pass rusher and run-stopper when healthy. Dee Ford, the team's first-round pick in 2014, played just 122 snaps in his first season and should get more reps as Hali's potential replacement. In a stroke of incredibly bad luck last season, Derrick Johnson tore his Achilles tendon in the season opener. Johnson will be back for the 2015 campaign, and that's great news.

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​3. Philadelphia Eagles: Philly's linebacker group didn't escape Chip Kelly's off-season roster makeover, but if everything goes as planned, this corps that was so good in 2014 could be even better in the new season. Edge-rusher Connor Barwin was the standout last season, with 14.5 sacks, eight quarterback hits, 24.5 hurries and six batted passes. Barwin is also a truly versatile player, capable of dropping to linebacker depth and providing excellent coverage. Kelly move No. 1 was the release of veteran pass-rusher Trent Cole, later picked up by the Colts. The Eagles will start Brandon Graham in his stead, and Graham might be able to finally outgrow his primary current legacy: that the 2010 Eagles took him with the 13th overall pick with Earl Thomas still on the board. Graham had 5.5 sacks, six hits and 23.5 hurries in just 499 snaps playing behind Cole, and a breakout season seems imminent.

Kelly's second move was the trade of LeSean McCoy to Buffalo for inside linebacker Kiko Alonso, who played for Kelly at Oregon and made his name as one of the NFL's best at his position before missing the entire 2014 season to a torn ACL. Alonso is a perfect fit for Billy Davis's expansive hybrid defense if he's 100% back to health as team reports have indicated. He's outstanding everywhere from run-stopping to coverage, and few (if any) inside guys have his range. Mychal Kendricks and DeMeco Ryans will tag-team as more pure run-stoppers with Alonso as the wild-card.

4. Pittsburgh Steelers: Pittsburgh's 2014 defense was an atypical disaster in 2014, but the linebacker group was pretty solid, and it started with James Harrison. The veteran started last season with a retirement ceremony, but with injuries to the overall linebacker group, former defensive coordinator Dick LeBeau called Harrison and asked him to come back. At 36, Harrison did just that to the tune of 5.5 sacks, eight hits and 13.5 hurries in just 432 total snaps. Harrison re-upped for another season and has said that his focus is more on football this season. In 2015, Harrison will likely split reps with Arthur Moats, with Jarvis Jones and first-round pick Alvin "Bud" Dupree on the other side. Jones has been one of general manager Kevin Colbert's biggest busts to date, but Dupree could really surprise–the star of the scouting combine can cover as well as he rushes the passer.

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5. Cleveland Browns: When the Browns signed Paul Kruger to a five-year, $40.5 million contract following the 2012 season, many assumed that it was a case of Super Bowl hangover. Kruger had been productive in Baltimore's stacked linebacker corps, the reasoning went, but how well would he do as the main point of focus on a lesser defense? Kruger answered the naysayers by registering 51 total pressures in 2013, and matching that total in 2014–the difference being that in 2014, he finished the season with 11.5 sacks, a sizeable improvement over the 4.5 he had the prior year. Opposite Kruger on the outside is Barkevious Mingo, who has as much purely ridiculous physical talent as any pass-rusher in the league, but is still trying to put it all together. Mingo managed just two sacks in his sophomore season, but racked up 25 total pressures and looked unblockable in short spurts. He'll have to compete with Armonty Bryant and rookie Nate Orchard for snap time on non-passing downs.

Karlos Dansby and Craig Robertson manned the inside for this group, and while Robertson was a pleasant surprise, Dansby was simply living up to the four-year, $24 million contract he signed before the 2014 season. The veteran is one of the better half-field linebackers in pass coverage, and he proved that again last season despite missing a month with a knee injury.

4-3 Linebacker Corps

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​1. New England Patriots: The Patriots run a hybrid defense that incorporates 3–4 and 4–3 principles, but given the quality of this trio—Jerod Mayo, Jamie Collins and Dont'a Hightower—it's possible that you'll see a lot of three-linebacker base looks from this defense in 2015, and we'll rate them as such. Belichick adapts scheme to personnel like nobody else, and this is the clear strength of his defense. Hightower had a bit of a breakout in 2014, leading all NFL inside linebackers with six sacks. He's Belichick's primary vehicle for the inside blitzes the coach is dialing up more often these days.

Usually, the loss of a player of Mayo's caliber would be a disaster for any team, but Mayo's shortened 2014 season—he missed 10 games due to a torn patellar tendon—allowed second-year man Jamie Collins to step in and become one of his team's best defenders. Collins can blitz and he's a great run-stopper, but his primary asset is his athleticism in coverage. Given the off-season weakening of New England's secondary, expect the Pats to depend on this even more in the new season. The projected lineup of Mayo on the weak side, Hightower in the middle and Collins as the strong-side linebacker is impossible to top among 4-3 linebacker groups in terms of overall talent and versatility.

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2. Carolina Panthers: It's not exactly breaking news that Panthers middle linebacker Luke Kuechly is one of the best, if not the best, at his position, and it's been his consistency through his three-year career that stands out as perhaps his most valuable asset. Kuechly does everything well: run-stopping, blitzing when necessary, and dropping into coverage. He's improved most in that last category over the last three years, merging his athletic gifts with outstanding field reads. Kuechly learned a lot of what he knows from Thomas Davis, the 10-year veteran who has recovered amazingly well from multiple ACL tears a few years back–in fact, he's the first player in NFL history to return to action after three ACL tears, and he's missed just two games in the last three seasons while playing at a very high level. Davis is even rangier in coverage than Kuechly, which makes it very easy for the Panthers to run nickel defenses. The third man in 2015 will be rookie linebacker Shaq Thompson, a bit of a surprise with the 25th overall pick. Thompson has ridiculous athleticism—he's been projected as a safety and played some running back in his final college season—but at 6'0" and 228 pounds, he had some wondering if he'd be ready for the physical nature of the NFL at the linebacker position. What those naysayers miss is the league's trend to faster, lighter linebackers over the last few seasons. Thompson fits that paradigm perfectly, and he'll be groomed to take Davis's spot over time.

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​3. Seattle Seahawks: The league's best defense may be defined in the court of public favor by the Legion of Boom secondary, but it doesn't go well at all without inside linebacker Bobby Wagner. The 2012 second-round draft picked missed five games in the 2014 regular season, and the defense fell apart without him. With Wagner out, the Seahawks ranked 20th in points allowed, ninth in yards allowed, 23rd in rushing yards allowed, 24th in completion percentage allowed, 20th in first downs allowed, 13th in fourth-quarter points allowed, and they went 3–2. When Wagner returned, and over the following four games, the Seahawks ranked first in points allowed, first in yards allowed, third in rushing yards allowed, tied for fourth in completion percentage allowed, ranked second in first downs allowed, and went 4–0. And that was despite injuries to defensive tackles Brandon Mebane and Jordan Hill. Seattle recognized Wagner's value with a four-year, $43 million contract extension in early August.

As much as the Seahawks play nickel (five defensive backs on 58.8% of their plays in 2014 per Pro Football Focus), these linebackers need to be versatile, and the tag team of Wagner and K.J. Wright are. Wright has played all three 4–3 positions at various times throughout his career, and outside of a slight issue when covering tight ends in the seam, he can do just about anything. Rounding out this trio is Bruce Irvin, selected a round ahead of Wagner in that 2012 draft, but with far less overall effect to date. The Seahawks have moved Irvin from LEO pass-rusher to hybrid linebacker, and he's still trying to figure it out. 2015 is his contract year, and could very well be his last season in Seattle. Kevin Pierre-Louis should see more reps in that third role this year.

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​4. Detroit Lions: The Lions agreed to a four-year extension with weak-side linebacker DeAndre Levy on Wednesday, and that's simply a smart thing to do on the team's part. Over the last two seasons, Levy has become one of the best overall linebackers in the NFL, though a bit under the radar. Levy's most obvious attribute is coverage, which he has to do a lot, because the Lions led the NFL in 2014 with defensive snaps of five defensive backs or more (76.3%). Levy can easily flow from side to side in coverage, and no linebacker was targeted more in 2014. He saw 99 passes thrown his way, and allowed just two touchdowns. He also led the NFL among 4-3 outside linebackers with 80 total stops.

On the strong side, Tahir Whitehead proved to be an asset. In 2015, he'll compete for the starting job opposite Levy, but he can also play inside, something he did very well last season after Stephen Tulloch tore his ACL last September while celebrating a sack of Aaron Rodgers. If Tulloch returns to form, the Lions could be running a lot fewer nickel packages in 2015. These are three linebackers you want on the field at the same time as often as possible.

5. Oakland Raiders: Oakland's 2014 draft was a pretty impressive haul for a number of reasons. The Raiders got their starting quarterback in Derek Carr, a great left guard in Gabe Jackson, and a defensive tackle in Justin Ellis with a lot of potential. But the star of the class was unquestionably Khalil Mack, taken with the fifth overall pick out of Buffalo. Virtually from the moment he hit the field in the NFL. Mack exhibited All-Pro potential. Those who devalued his effect because he finished his rookie campaign with just four sacks weren't watching the tape. Mack also finished his season with 30.5 quarterback hurries, the sixth-highest total in the NFL. And he was an absolute rock star against the run. Per Football Outsiders metrics, he was involved in 60 run plays, had a stop rate of 80 percent, and allowed 1.9 yards per carry. New head coach Jack Del Rio may line Mack up as a pass-rusher more often, which means his sack totals could spiral up as well.

Sio Moore was overlooked in all the Mack mania, but he acquitted himself very well, especially against the run. To fill their linebacker trio, the Raiders signed former Falcons and Saints linebacker Curtis Lofton to a three-year, $18 million contract to man the middle. It's an interesting decision given Lofton's subpar 2014, but perhaps Oakland's coaching staff is counting on the idea that it knows something the Saints didn't.