Semi long read but let me know if you agree
1. Jon Jones
Case For: His resume is pretty much unmatched (maybe GSP is close). He dominated what at the time was the UFC's glamor division. Rampage, Shogun, Machida, Rashad, Bader, Vitor, Chael etc. Then beat the next era of guys mostly cleanly (close fight with Gus in the first) . Pretty much only guy he did not beat that he could have fought was Rumble. Then beat ANOTHER era of guys in Smith, Santos and Reyes albeit perhaps should have lost the Reyes fight.
Resume unmatched. In his prime, pretty much as dominant as it gets.
Case Against: Steroids. Pretty much it. And since he is so high profile he might have access to high level roids no one else does.
Why the case against doesn't matter: I don't want to poo poo the sport. But it's pretty naive to think most of these guy at the top level aren't on something. But I guess who knows. Jon certainty has been dumb enough to keep getting caught. But it's more than likely with everyone is on something and so it's a pretty even playing field.
2. GSP
Case for: Probably the second greatest resume of all time. Dominated three generations of welterweights. Beat some of the greatest fighters ever, and had a stranglehold on his division for many years. Did pick up the MW title later in his career vs an also shopworn Bisping. Take that for however you want. In addition, most of his fights even against top guys were not particularly close.
Case against. Loss to Matt Serra
Why the case against doesn't matter: It's obviously a silly loss, but I would tend to agree with the notion that GSP's prime did not occur until probably...2009ish? I mean at the time of the loss to Serra he had 14 fights. Anderson had multiple losses 14 fights in and would have another to Chonan soon after, Khabib had been fighting cans etc. And with the brutal rematch that took place, not sure it should matter as much
3. Anderson
Case for: Probably the most spectacular fighter ever. Probably MMA's version of Roy Jones although I suppose to a lesser degree. Had a stranglehold on the middleweight division for a long time dominating excellent fighters like Vitor, Nate, Rich, Maia. And beating top notch guys like Chael, Okami and Forest. Pound for pound probably the greatest watch ever in MMA
Case against: Steroid issues, resume issues, silly losses pre and post prime.
Why the case against doesn't matter: It can be argued that he only took steroids post the leg break. Not sure I agree with it but he didn't pop beforehand so could be true. The resume issues are tough to overcome in this sort of debate because he obviously doesn't have the resume of Jones or GSP. BUT can be argued he won in a more spectacular fashion than either. And losses pre and post prime are rather unimportant imo. Now when did his prime actually end? That's more of a question (was it before the first weidman fight>)
4. Fedor
Case for: The Greatest Heavyweight ever. Dominated in a promotion that featured the best heavyweights in the world at the time. Beat one of the 5 best heavyweights ever (Nog) multiple times, and beat top 15ish (maybe top 10) guys in Crop Cop, Sylvia, Arlovski etc and fringe guys like Coleman, Herring etc. Usually in quick blowouts. Well versed on the feet and on the ground.
Case against: Post Pride suffered multiple bad losses. Possible resume issues. Never appeared in the UFC
Why the case against doesn't matter: Well at the time that he was in PRIDE he pretty clearly was the best Heavyweight in the world. And it can be argued that around 2007ish his prime was coming to an end. So by the time he came to America in Strikeforce he was past his peak. Maybe doesn't explain the Werdum loss but could explain the ones post that. Some resume issues when stacked up with these other guys but basically beat everyone in Prides. And never appearing in the UFC is no big deal if your resume is good enough (maybe his is. Up for debate).
5. Khabib
Case for: The most dominant Lightweight ever. Destroyed a number of really good fighters. An excellent title run once he got the belt beating Conor, Dustin and Justin in total blowouts. A few other quality wins in Dos Anjos, Barboza and Healy. And some decent guys like Tibau and Trujillo. Only close fight was against Tibau and once he really got going didn't lose too many rounds in his career.
Case against: It's basically impossible to argue Khabib along these other guys from a resume standpoint. Probably also missed out on a few guys he could have fought but did not (Cerrone, Ferguson, Pettis, Bendo, etc)
Why the case against doesn't matter: He was so dominant when he fought it's easy to say he probably beats the guys he did not get to fight. He was that dominant. So resume shouldn't matter as much here.