Going strictly by records, the best fight of all time would be Diego Sanchez vs. Phillip Miller, presuming Sanchez would agree to move up in weight and Miller would agree to fight a skilled opponent (he's shown reluctance in the past). Both fighters are 16-0, tied for the best current record in MMA history.
Perhaps stats aren't everything...
Well... best in MMA anyway because nothing in MMA is going to measure up to Ali Frasier 1 and 3 for a VERY long time.
I actually don't know if MMA really allows for that kind of dramatic contest. MMA fights end very explosively, so it's harder to get the see saw battles where two men destroy each other over 15 rounds.
Those fights are rare in boxing, but even harder to get in MMA.
wtf?
Diego Sanchez vs. Miller as the fight of the century?
what about Royce vs. Sakuraba?