I've played Ultima Online, Dark Age of Camelot, Anarchy Online, Everquest, World of Warcraft, Earth: Above and Beyond (was that right?), Asheron's Call, Star Wars Galaxies, City of Heroes.
World of Warcraft and Ultima Online are in a tie vote for me. WoW has that incredibly immersive and totally addictive gameworld that will continually be expanded and updated. I'm still paying. But, in a nutshell - it's like all other Blizzard games from Warcraft to Starcraft to Diablo and Diablo II - completely addictive like crack for 6 weeks then you're over it - except WoW lasts maybe 4 months. Then the leveling up gets as goofy as Everquest. Get a level up, get better equipment, fight tougher monster....after 4 months the next step is to YAWN
Ultima Online ties. The graphics are not as good, that's as obvious to me as it is to any 3 year old. However, the community is better. There are fewer kids, and more adults it seems. There is a wiser community more concerned about story and gameplay. There are folks for whom graphics are not as important but playing and living in a virtual world is - and being friends - is. There is an advancement system that IS NOT BASED ON LEVELING UP thank god, but is more sophisticated - tied to a wonderful vendoring type system for crafters who can have a blast without having to kill a thing. And there are far more items to obsessive over thanks to UO's 8 year history than any other game. For you fighters, there's a larger variety of creatures, characters, and "paper verses scissors" than any other game...after all, that's what all RPG fighting is about - a carefull choreographed and fancy looking "paper verses scissors".