Although this game doesn't count, I'm going to say "Neverwinter Nights"
I have had bad experiences with MMORPGs. Illusia, The Realm, UO, and then EQ...all gave me nightmares.
However, NWN has one thing that simply drives me CRAZY...
The ability to craft my own AD&D Modules, then Run them online, and play a DM as people join my world.
Imagine being able to inhabit an NPC and have a conversation with someone who entered your game and they don't know if its the game, or the DM.
Imagine inhabiting a monster, and then BEGGING the party that joined your game not to kill you. Imagine the depth such a system would allow.
Now, they tried this with Vampire the masquerade, and although the idea was right, the implementation was all wrong.
From what I've seen, Neverwinter Nights looks like it will be my obsession.
I already have the story written, and as soon as the game comes out, I'm making the module. Five of my ICQ AD&D fans have already offered to try it out once its done.
I can't wait....