Conor McGregor is back. After more than 20 months without an MMA fight, the former UFC lightweight and featherweight champion will take on Khabib Nurmagomedov on October 6 in Las Vegas. The Dagestan native, in possession of the lightweight belt, has won all 26 fights of his professional career.

Every McGregor bout is big deal. But this -- against one of the most talented fighters in the sport's history, someone the Irishman has legitimate beef with -- feels different. Is it the biggest fight in MMA history? We asked our panel -- MMA reporters Brett Okamoto and Ariel Helwani, MMA editor Jeff Wagenheim, SportsCenter host Phil Murphy and digital contributor Chamatkar Sandhu -- for their takes.

Okamoto: I believe it is. I think it's easy to forget how massive McGregor's featherweight title fight against Jose Aldo was in 2015, because of how big the two Nate Diaz and Eddie Alvarez fights were in 2016 -- but don't forget that World Tour before McGregor, Aldo, and how enormous that was for his star power. That fight generated a $10 million gate at the MGM Grand, and (by my estimation), drew the largest Irish fanbase of any of McGregor's fights. That, to me, is probably the second-biggest fight in UFC history, behind this one.

Of course, the Diaz rivalry was huge for both fighters and the UFC brand. But Nurmagomedov carries the weight of a Russian and Muslim fan base, and McGregor has never been bigger after last year's boxing match against Floyd Mayweather. Each of McGregor's past four fights have been huge, and Chuck Liddell vs. Tito Ortiz 2 in 2006 was a big one for the UFC -- but by the time October 6 rolls around, this fight will be bigger than any that came before it.

Helwani: I personally think this is a no-brainer and would love to hear from anyone arguing in favor of it not being the biggest fight in MMA history. It's pretty simple, really. You have the biggest draw in the history of the sport, the most famous fighter in MMA (and arguably the most famous fighter in all of combat sports) as well as one of the top 10 most famous athletes in the world, returning after an almost two-year hiatus. He's competing against perhaps his biggest rival in a fight that has been brewing for almost two years. It's personal. You have the footage of the bus incident, you have the footage of Conor in handcuffs and it's team vs. team. It's authentic. People are picking sides. You can't script anything better than this. You have the most dominant force in the history of the sport, Khabib Nurmagomedov, who is 26-0 and many believe has never lost a round, going up against a guy who has been a locomotive for the majority of his UFC career.

It's also a throwback fight because you have the classic grappler vs. striker matchup, so everyone is curious as to who will be able to keep the fight either standing or keep the fight on the ground. This is theater of the highest kind. I truly believe that it will sell close to, if not more than, two million pay-per-views. It's the perfect mix of stardom, anticipation, promotion, reality, passion and emotion. It's a tornado of all of these feelings. At the end of the day, distance always makes the heart grow fonder. The sport misses Conor. His return against anyone would be a gigantic fight. Throw in Nurmagomedov, the backstory, and it makes it the biggest fight of all time. Easily.

Khabib Nurmagomedov is coming off a decision win against Al Iaquinta last April to earn the UFC's lightweight belt. Jeff Bottari/Getty Images

Wagenheim: Define "biggest."

If we're assessing Nurmagomedov vs. McGregor purely as a fistfight, there's certainly intrigue on several levels -- striker vs. grappler, one guy unbeaten and the other a two-division champ, etc. But we've been here before with Chuck vs. Tito, Georges St-Pierre vs. BJ Penn and a few other blockbusters. Bottom line, the first meeting between Jon Jones and Daniel Cormier -- both ostensibly unbeaten -- is hard to outdo.

If we're evaluating in terms of big-picture significance, the gold standard has to be Forrest Griffin vs. Stephan Bonnar, right? For years, we've been fed the narrative that the Spike TV telecast of the Season 1 finale of "The Ultimate Fighter" saved the UFC from financial ruin. Would the whole circus tent collapse without this big October 6 showdown?

But Khabib vs. Conor takes "biggest" in several directions all at once. It's a tantalizing matchup that could go either man's way. There's the international, clash-of-cultures flavor. There's the unsurpassed star power of McGregor, augmented by the absence-makes-the-heart-grow-fonder appeal of his return. There's the acrimonious backstory that, with its police reports and court appearances, makes the Jones-Cormier press conference brawl seem like child's play. And then, of course, there's the pot of gold that'll be waiting for the UFC at the end of what likely will be the biggest pay-per-view in the promotion's history.

So biggest? It depends. But we can safely say that this one has the broadest appeal of any fight in MMA history.

Murphy: To avoid recency bias, you're skeptical any time the new, shiny thing is called "the biggest ever." But Conor McGregor has made a habit of rewriting MMA history. His marketability is unprecedented.

"Biggest" is subjective, and measuring it is a challenge. The gate record set by Alvarez-McGregor at UFC 205 in New York City won't fall in Las Vegas. I measure "biggest" by buzz, and I make buzz tangible not with live attendance but with pay-per-view buys.

The gold standard in that regard is Diaz-McGregor II at UFC 202: an estimated 1.65 million. After a two-year MMA layoff bridged by a boxing match against that sport's pay-per-view king, casual demand for McGregor may be at an all-time high. As an underdog against the unbeaten Khabib Nurmagomedov, public intrigue would likely grow further.

A world tour of press conferences to promote the fight would topple the PPV record. UFC president Dana White says that's not in the cards, so I think Khabib-Conor drops short of the mark. For that reason, it's not the biggest MMA fight ever. But it's close.