well, I've played a lot of them, and i mean a LOT. The only two I've been able to stick with for a long time is WoW (World of warcraft) and Eve online. Yea i know there are people who hate WoW but overall it has been able to overcome all the other mmo's out so its doing something right i guess, and EvE is different. Its a space game and its more about thinking than clicking buttons. If you want an mmo check em both out if you already haven't seen them.
Ragnarok Online has been my all-time fave. I played a lot of MMORPGs and yet, I keep on coming back to RO. Atm, I am playing Uncharted Waters Online. Started a month ago. With its interesting and strategic gameplay, i think imma be playing this for a long time.
My favorite MMO RPG was fantasy earth zero which got shut down. At least the japanese version is really popular (Its like in the top 10 list of overall best games.)Now I just want to play it again.
My favorite MMORPG today is Uncharted Waters Online because of it's nautical theme and it's strategic sea battle. The fun part of it was there's a lot of quest and lots of places to discover. Also I've learned a lot of world history.