Between 09 and 05 for me.
09 - UFC having one of its biggest years with 100, Brock as HW champ, prime GSP and Anderson, TUF US vs UK culminating in the H bomb etc. BJ was still considered legit. Randy was still having a decent old man run etc etc. Outside of the UFC you had Affliction, well..dying, but burning very brightly first. Strikeforce signing Fedor out of nowhere and all of a sudden looking like a serious player, WEC was still around, Reem was in peak uber form, Fedor hadnt lost yet etc etc.
05 - Pride was white hot with no indication it was gonna die in a couple of years, K-1 was still going strong with MMA fighters in kickboxing matches and occasional MMA fights, the UFC was really gaining traction due to TUF.
2003 all the way through to 2011 was awesome for MMA tbh. There was mini peaks within it but it was ALL great. Sadly it will never get that good again because a lot of the elements that went into creating that perfect storm cannot be recreated anymore. You cant ever bring back that middle ground of the skill level being solid but there still being a crudeness and excitement to things. In 2019 MMA feels like it reached its ceiling a while ago already when it comes to both skillsets and popularity. I think JDS/Cain getting on Fox was kind of the end of that peak era. It was the "we made it , ma" moment. The culmination of all the momentum the UFC had been building for years before that.