I've played allot of MMOs over the years. My first was Dark Sun Online: Crimson Sands technically. Certainly I played a few MUDs back in the day, but I could never agree that they were anything like a graphical interface multi-user game. UO was however probably my first and most memorable MMO experience. UO had something that really seemed to allow a person to suspend disbelief. It took on a sort of life that while I was playing blotted out reality in that way that only really compelling novels and movies can do. I developed an identity for my toons and it became what I was while I was online. The relationships that I developed with other players weren't mine, they were my toon's. The role-play was reality, at least for the time spent in game.
I tried Everquest when it came out, but my hobbit like toon drowned in the first 20 minutes of play and I stopped playing. At the time I had been watching my best friend beta test Asheron's Call and I just could not wait for the game to come out. Asheron's Call, like UO seemed to possess some force, or magic that made immersing yourself in the game a simple matter. Logging out was akin to waking from a vivid dream that you did not want to forget. When I was playing Mitsi in Asheron's Call, I was essentially her, and her friends in-game were her's, not mine. When I logged out I went back to my life, real friends and my job. The game allowed me to escape to a different place and it was nothing like reality proper. That experience has yet to be found again since leaving those games.
After Asheron's Call 2 demonstrated its failure in beta, and DAoC started to lose its luster there really wasn't much out there that could bring back the magic that UO and AC possessed. Shadowbane came pretty close, however it became a tedious affair post level 40. World of Warcraft had many things going for it, and certainly I played it for many years, but it has never been able to get me to feel for it, to truely care about my toons and the world they inhabit. It was merely something to do for a few hours each day, and someplace to chat with people that seemed to share the same basic interest. However we all accepted that we were people going about our in game business, and not a band of surley Orcs hell-bent on collecting Night Elf ears as trophies.
I've tested, purchased and played just about every MMO since, even the Asian variety that generally end up as shallow attempts at capitalizing on basic greed. So far not a single one has truely been able to awaken that realism and engrossment that the earlier ones did. I'll be honest, it may not be the games themselves. I'm sure that I have changed as a person as well. However, I should like to think that I would recognize a really immersive experience when I see it. (Mass Effect 1 vs Mass Effect 2 for example?)