I am a HUGE Maplestory fan, I have played it since it first came out, for the KoreanMS beta, and have tried to quit it, seriously, about 50 times, but always find myself coming back to it. I would actually say it is no long a game I just enjoy, it is something that has become somewhat of an unhealthy addiction.
My favourite game would have to be Ultima Online. I've been playing it for eleven years, and I still find myself going back to it often. Another game that I really like is Arcanum, which is definitely the best single-player RPG of all time.