1. WOW - Sorry but it has literally almost everything an MMO needs.
2. SWG - Even though I hated the way the game turned out and I'm glad it's gone now. Still some of the best memories.
3. Guild Wars - PvP'd my butt off in that game.
4. SWTOR - I love Star Wars, wish it had more PvP in it but meh, what can you do.
5. The Repopulation - Haven't even played it but it's just an MMO I'm looking forward too. Didn't have a 5th favorite.