Runescape Classic till it died, then FlyFF till I got sick of grinding. Hmm, I'd say Tibia is pretty cool if you have friends, and DarkEden was pretty interesting till it got killed by hackers. Oh yea, and GunZ... till it got killed by hackers. And even older, freakin' Ants on MSN Games! ...Till it got killed by hackers.
I've played countless mmos in the past. Most of them I quit because they felt meh (ex. WoW), but some just got shut down (ex. Exteel).
I used to play Space Cowboy Online, which was my favorite mmo up until then, until that closed down and everybody went over to AirRivals. Then I was active in that for a little while until like 4 years ago.
Still the best PvP to be found in ANY game I've played so far, but many other features such as the grind, cash shop, and interface are shit.
Right now I'm sort of active in Vindictus and Tribes: Ascend.
Waiting on seeing how Hawken, Blade&Soul, and some other mmos turn out.
Might check out Tera and GW2 when they have like 7 day free trials.
Of the two dozen or so MMOs I've tried, Wizard 101 might be the best, although it is rather little-kiddish. WoW, of course, is undeniably pro. Dofus, Maplestory, and Perfect World were all good enough to keep me playing for at least 2 months. A Tale In The Desert gets an honorable mention for uniqueness and crafting focus.
Things that did not quite make the cut: Eudemons, Flyff, Luvinia, BrightShadow, MixMaster, Guild Wars, Runes of Magic, Earth Eternal, zOMG, and some more I'm too lazy to look up the names of.