If you're going to go "Best Ever", you have to include other MMA fighter/ organizations from history. Here's my list of 'woulda, coulda, shoulda' fights, and including a few for today as well. (Not in any order):
Rickson Gracie/ Sakuraba (~2000)-- everything Royce was doing in the US Rickson was doing in Japan. This would have been crazy
Liddell/ Hendo (2006-7): Would have been fun for Hendo to get a tune up, or cage warm up against one of the greats.
Fedor/ Couture: (2008): Couture had just come back and won the HW title... would have been fun... and people would have been able to see the REAL Fedor.
Couture/ Wandy (2003): Randy just beat Liddell... why not put him in there against the person with the most consecutive title defenses in MMA history?
Mir/ Arlovski (2005): Would have been cool to see Sambo vs. Jiu Jitsu when both were at the top of there game (Yes, I know Mir is making a STRONG comeback)
Cro Cop/ Arlovski (2005): Yes, I know Andre's chin is now known to be made of glass... but in 2005 both these guys looked unstoppable.
Wandy/ Shogun (2005): Teammates then. Good friends, but the 2 BEST LHW's on the planet at the time... period.
Fedor/ Barnett: Anytime. This would have been amazing.
Gomi/ Melendez (ASAFP): These two would be dynamite from start to finish.
A. Silva/ Paul Filho (ASAFP): Dana SIGN FILHO NOW!!! Filho is undefeated, sick jiu jitsu... Silva has a HUGE advantage standing.... but when doesn't he?
Hendo/ Marquardt (2009-10): Would have been an interesting matchup. Fun to watch against two VERY respectable opponents.
I also like Frank's Spider/ Machida matchup, and someone else's Couture/ Machida.