Out of the MMOs that I've played, Guild Wars was definitely at the top of the list. The overall design was awesome, the gameplay was fun, the fact that it was free monthly, and (when I played it) the level cap was only TWENTY! Not to mention you could build a ready-made level 20 character. That meant you didn't have to spend AGES levelling before you could do any PvP.
But for some strange reason that I STILL don't understand, I logged many more hours on World of Warcraft, even though I hated it throughout. It was so damn addicting. Thank god I got out of that rut a few years ago.
In terms of overall favorite games:
Final Fantasy Tactics
Ogre Battle 64
Super Mario Bros. 3