Every expansion we hear "They ruined [spec/class] for me" or "Blah Blah expansion was the best for [spec/class].
Thinking of one spec (Probably the one you mained the longest.) in which expansion did you feel was the best for that spec/class?
(Feel free to add addition classes in a comment but vote for one.)
For me for Ret Paladin would of been Wrath, as it was the beginning of Retribution being a comparable to the other melee dps specs. I did good numbers in raids, the survival-ability was great in pvp yet still packed a punch. towards the end of Wrath Ret made good tanks for quick heroic 5mans.
For Warrior my favorite moment was Cataclysm not only for the numbers we did in Raids but Fury PVP was so fun, out of all the expansions I enjoyed PVP the most.