Its GSP or Jones based off their championship reigns and their strength of schedule. I give a nod to GSP, I think people dismiss how great some of his wins were due to several factors (like him not being much of a finisher or the fact that he came to prominence much earlier than Jones so we've seen all the guys he beat become washed up already skewing perspectives).
Guys like Thiago Alves pre brain injury smashing Koscheck, Hughes and going on like a 6 or 7 fight winstreak to the title. A killer striker with amazing takedown defense and the cardio to go a hard 15 minutes shown. Shields was a top 10 pound for pound guy in his prime with a legendary resume (Okami, Woodley, Maia, Hendo, Sakurai, Lawler, Kampmann, Daley, Condit). Fitch had an 8 fight win streak with multiple top 10 wins going into the GSP fight and continued to be impressive for years after the GSP loss too. Hughes is arguably the second best WW champion of all time and GSP beat him twice, still in Hughes prime. Penn largely considered the second greatest LW of all time fought GSP twice in his prime as well, people say he beat up a LW...How many MWs did Jones beat up, morons? Chael? Vitor? Rashad? Machida....All fought at MW, shit Shogun easily could have too. And the Hendricks decision? I scored it 48-47 GSP, it was definitely controversially and Hendricks won his rounds way more convincingly but fucking Jones has just as shakey decisions vs Gus, Santos and Reyes...Hendricks also was unbelievable on his run to the title, the KO of Fitch and Kampmann were insane, he beat Koscheck and Condit. Diaz was on like a 12 fight winstreak and everyone had him in the top 10 pound for pound when he came to the UFC too. Condit beat tons of top 10 guys enroute to his title shot vs GSP, is a legend and was the last WEC champ (Ellenberger, Rory, Hyun Kim, Diaz) if you werent watching MMA back then to know how good these guys were, your opinion means so little. Remember, wait to evaluate guys like Usman, Colby, Edwards etc until the sum of their career is up and theyve fought years at a championship/contender Level...these guys have 15 pro fights lmao, wait until they have 25-40 pro fights, experience lingering brain issues, have major wars, knee surgeries etc.
Just, GSP to me, beat the best opponents, gave up the fewest rounds, won the title, lost it, got it back and defended it 9 times them retired. His return vs Bisping means little to me, if he defended vs Whittaker and won I'd have dropped my jaw though.