For me, its actually a few, hard to choose between them.
Asheron's Call, only because it was my first MMO so it held a special place in my heart. A truly unique MMO that could been classified as a sandpark and showed that you can make a game without forced grouping while giving a reason to group (which many did)
next never left beta, but it was amazing for just a beta. I was in beta and I'd have been playing that instead of WoW, granted it did have similar WoW-esque mechanics to it...
Not many heard of it or got a chance to play beta, but it was amazing
Next I'd say probably Auto Assault and Tabula Rasa. AA never really got a chance, but its a game that still hasn't been re-made. Tabula Rasa showed you can actually have a good AI in an MMO without it being brain dead and standing in one place.
What about for you guys/gals?