I'm only playing two really, and I honestly can't put one above the other just because they're both so amazingly awesome (and totally different genres) so: 1: World of Tanks 2 on list but technically still 1: Rift
Thinking of checking out Rift...soon mabye,don't want to put too much on my table.Having lots of fun with AION great game!!!
Honestly, I could not stand Aion. It became really repetitive to me, and in general the Korean type fantasy games just don't appeal to me. To make way for Aion they also shut down Tabula Rasa, which I'm still fairly miffed about - but I digress.
You really should give Rift a shot. It basically takes elements from almost every MMO, refines them, then slaps them all into one game and adds its own unique things, to boot. The actual rifts in the game can open just about anywhere on the map, and are essentially public quests that reward groups working to close them faster for better loot, but can usually be done solo. Footholds also spawn around the map, and in addition to spawning an elite mob with normal mob escorts, have their own elites to defend them.
Then there are whole zone changing Invasions, which basically force everybody in a zone to work together to finish their phases, then defeat the boss that spawns for very nice, rare loot. PvP combat and warfronts are a lot of fun and give a pretty good wealth of gear of their own. Can also get the PvP gear through doing Rifts/Footholds/Invasions, which is nice.
All in all, I'd say that if you like WoW/Aion/pretty much any MMO, it's worth looking into. I played from the very first closed beta events, and it's... I'm amazed I'm saying this, but aside from the occasional crash from alt-tabbing out of the game too often, I have had no issues with the game, and encountered no bugs. Even at launch the servers were never overloaded! That, plus the developers at Trion and the GMs being incredibly kind... If I ever get bored of the game and stop playing, I'll keep paying for my subscription, because these guys flat-out deserve my money.