Out of the ones I've played, I'd say Aion. Simply because it was a beautiful game with great classes and gameplay, lovely scenery and aesthetics completely ruined by an impatient community who wouldn't wait for the devs to fix a few simple problems at endgame.
I'm still pretty convinced that SW:TOR failed because of the community all getting WoW hankerings again. The classes are great, the stories are really compelling, the cut-scenes are really well done and for a new game there was plenty of content.
I think the majority of MMO rerollers are planning a mass exodus away from WoW only to realise that they miss it, then come back and drag the whole community down on their way out with a hundred forum threads of "why X is dying." People will always insist on steamrolling their way to the level cap and then whining about the lack of content in launch week.