Crocop vs Fedor was the biggest, but I don't think it was as compelling, or that it was the clear #1 vs the clear #2 at the time. Not Fdor's fault, but he didn't really have anyone when he was at his best, at least after he dispatched Nog, who has flaws equal and opposite to Crocop. It wouldn't have been interesting to me to see Fedor fight Arlovski/Sylvia/Mir or Barnett in those days, either. I mean, I'd have been interested, but it wasn't ever going to be two Gods fighting in my eyes, in Fedor's reign
I liked Lesnar/Velasquez in terms of 'who's the toughest guy on the planet', and it finally ended the Lesnar myth. It also broke him as a fighter, fortunately and unfortunately (hate the man, love the game he brought)
Rampage/Silva 2 was likely the greatest LHW fight of all time at the time, and it lived up to expectations, and then some. They both seemed capable of being HW champs, too.
The lighter weight classes don't hold the same appeal for me, and there hasn't been something as epic as those, in my mind.
But the very greatest fight - in terms of drama, meaning and the abilities of the fighters was Gus v Jones. Jones looks able to be HW champ, and is the greatest LHW. Gus beat him up all over the place, stuffed takedowns, etc. Jones responded with incredible heart and determination, to which Gus showed the same in surviving once hurt. The 5th round was two dead men walking.
It remains to be seen just how great Gusto will become, or if he is just Vernon Forrest to JJ's Shane Mosley - but, if he can beat Johnson and Jones, and defend once, then a third match with Jones would be vs the two greatest LHW's of all time. After which, I would hope they'd try their hands at HW.