For me, I'd have to go with WoW. Even in the state it's in right now, I can still call it my favorite. No other game will live up to it, simply because I've been playing for over half of my life and Azeroth is like a second Earth to me at this point. I don't currently play right now but I have hopes that Blizzard will get their shit together for the next expansion and hopefully it will deal with Queen Azshara.
Other than that, Maplestory holds a huge place in my heart but I can't really bear to play it anymore.