d.c. universe. played it pretty hard core for about a year on ps3 and i still go back occasionally. i love that it's more of an arpg compared to the typical point and click mmos --- PSN:Dragonlibrarian Steam:Dragonlibrarian7
I've actually been really wanting to go back to DCUO. But I always get stuck at the create-a-character stage, because pretty much every name I can think of is taken. And I'm awful at thinking up semi-original super hero names (that I'd actually use for my own character, anyway).
That said, probably World of Warcraft. If I could do it for free, I'd totally go back and revisit my old characters, and level something new, to see what the game's like today. --- The sole purpose of this space is to make my post look longer.