Even though I haven't been playing much of it, I'll have to say Runescape. I've grew up on the game, and I have a lot of great memories. I'm trying to delve into other MMORPGs in the near future, too. Hopefully I can find a new game I enjoy playing.
Even though I've quit, I've gotta still say RuneScape based on the fact that I played it for 9 years, pretty much spanning my entire childhood. I play MapleStory now and I won't lie it's a close second as I've only been playing it for around 7 months.
Defiantly lineage 2. Has great gameplay and EPIC pvp (especially in parties and castle sieges). Not to mention it recently became 100% free (no subscription, etc). Only con is that it is a grindy game but I think it's really rewarding.