GSP would destroy any of them. GSP would also destroy khabib
In their respective primes, I would agree - but GSP was never one to destroy his opponents after his Matt Serra loss. He overestimated Matt Hughes in their first of three fights, and lost, then underestimated Matt Serra in 2007, and lost.
He went from legitimate welterweight bruiser to cautious champion, wishing to retain his title.
I find it interesting that GSP and Ronnie Coleman had similar legacies...GSP won the title originally in 2006, but ultimately got it back in the Matt Serra II fight, and was champion for 5.5 years straight. He could have told the UFC he was going to keep the title for the 12-month allowed period, and ultimately relinquished it in November of 2014, making a 6.5 year title run. Plus another seven months [IIRC] as champion after the Matt Hughes II fight. Instead, he technically had 5 years and 7 months as champion, and another 7 months between the Hughes II and Serra I fights. So that's 6 years and 2 months as welterweight champion. He also could have kept the Middleweight title for 12 months, but I think he felt it was disrespectful to hold up the division for something like 20 months to make that fight happen, so he decided to relinquish it right away.
GSP and trainer John Danaher have a way to lie without lying. GSP doesn't like to lie. On Rogan, Danaher said that GSP didn't cherry-pick the Bisping fight, but only took it because GSP was good friends with Chris Weidman, and didn't want to fight him for that reason. Ok, cool. So how about Rockhold? What reason did GSP have for not fighting him?
And relinquishing the title immediately was, to me, evidence that GSP had no intention of defending the middleweight title [the first defense would have been against Robert Whittaker]. And rather than admit this, he did the respectful thing by relinquishing the Middleweight title to allow the division to move forward after his comeback put it on pause for 20 months. So if you do the math, GSP only held up the division by eight months [although an argument could be made that Bisping was given the Dan Henderson II fight in part to ensure a Bisping win, and a GSP comeback fight against him...but you could also argue that was just Henderson's farewell retirement fight. GSP accomplished getting out of his Middleweight contractual obligations by claiming some vague digestive tract disorder [colitis] which could easily be faked just by making yourself so constipated that you sh*t blood.
Sometimes I wonder how many UFC fighters are on painkillers...and GSP would have likely had a legitimate reason to use them after his last two fights. Maybe he just spent a few days on prescribed oxycodone during his recovery in order to sh*t blood, and get a Quebec doctor to write him a note for suspected colitis, which would be a valid reason to bail out on his contract.
But I'm just speculating here.
Back to Ronnie Coleman, he technically held the Mr. Olympia title for an 8-year run, until the night in 2006 when Jay Cutler won...GSP and Ronnie are both 5'10.5", long-time champions, and arguably, the GOAT's of their respective crafts. Although GSP is ultimately a weight class champion...I think he could beat at least half of all Middleweights, and maybe 25% of all Light-Heavyweights...but I can't imagine him beating the overall champs in either of the heavy classes.
Back to your point about GSP vs. Khabib - I think in their respective primes, GSP could beat him by decision, and probably even finish him if he decided to be opportunistic, like he was with Bisping. Khabib is really on a whole other level though, and I'm not 100% certain GSP would run through him or anything. In their primes, maybe...it's just that Khabib is such a freak, that I wouldn't even want to make that bet.
A 2021 GSP vs. Khabib fight would be another matter...GSP will be 40 in May, and Khabib would still be 32 by then...that age disparity might just be enough to give Khabib the advantage...IMO, that fight, at 40 years of age, is too risky for his legacy, and GSP cares a lot about his legacy. I'm confident that he would rather win and make no money than lose, and make millions [the way Conor lost to Mayweather, and made something like $100 million in the process].
With a decent amount to gain from beating Khabib, but a LOT to lose, legacy-wise, I'm not so sure GSP would take that fight. The way he talks about it when asked...it seems that fight won't happen. And back to Ronnie...look what happened to him at age 42 in 2006...he lost to Cutler. GSP is just at the point now where he's going to start declining physically, even though his knowledge is going up.
[sorry, AGAIN, for the long post...I'm a big GSP fan. I'm Canadian, eh?]