...the most important/historical/hyped/biggest MMA fight of all time? What other fights could make a claim for this title?
In terms of who the fighters were, what the fight meant, how good they were at the time, how good they went on to become, how they performed on the night, what the fight was like...
...I would probably agree that
Fedor-Crocop is the major fight in MMA history.
The only other realistic contenders I can even think of would be
Fedor-Nog III(#1 HW ever vs #2 HW ever),
Anderson-Hendo (UFC #1 P4P vs double Pride champ),
Hughes vs GSP II (though GSP didn't quite have the credentials then), and
Chuck vs Randy III (trilogy decider between two UFC legends).
GSP-Penn II was a genuine superfight, but I think the fight itself and his later exploits showed that BJ was not at his fittest or his best as a welterweight, whereas Fedor-Crocop was a meeting of two superpowers at their best.
Chuck-Wandy was a superfight too, but it happened too late to merit consideration here. The UFC Wanderlei was a pale imitation of the man who had ruled Pride years earlier.
