BC - Karazhan
WotLK - Raids puggable (yes really), 10/25, hc mode model, Halls of Reflection maybe?
Cata - Heroic Dungeons improved in difficulty again. The guild forcing actions blizz took also made me join a nonsrs guild, which turned out to be pretty fun in the end. Tol Barad Peninsula pvp.
MoP - since I got permabanned before even firelands patch in cata... can't say
I totally used one single thing for each xpac ther.