Currently, I'm not playing any MMOs, I can't help but feel the best have already been and that much of what is out there now, just seems so...derivative.
I cut my teeth on EVE-Online for around 5-6 years, everything from PvP alliances through to PvE safety and casual play, as well as specific role play groups. From there I've tried WoW, EQ, SWG and Star Trek Online, but I've always felt that the constant grinding and the lack of depth to the backgrounds/npcs in these games just leaves them boring. I mean, WoW I just couldn't take seriously when you go to a "city" that takes 2 minutes to walk around, it is just such a shallow setting that really wastes the depth they do have in their writing. Also, what's with everyone running all the time, I know it's practical, but it is so damn immersion breaking.
Unfortunately, I've learned over the last decade that the type of game I'd consider to have enough role play depth, most people just wouldn't want to play, because the majority of the player base actually seem to like that grinding crap.
Other than that, I have been, on and off, active in Second Life for a long time, and while I wouldn't consider it an MMO, it does at least provide an alternative role play outlet, though the quality can be extremely variable and you are very much reliant of the quality of what players can build and the limits of their vision and skills accordingly, which makes it all rather hit and miss finding something that can keep your attention.