Ragnarok Online was also a long time game of mine as well.

Atlantica Online was also a pretty neat game that got me engaged for awhile.

S4 League is definitely addictive if you like MMOTPS games, I seem to find myself getting drawn back into it.

In the end though, if you're looking for a long term game, it's just like what the others were saying. Find a game you like, find some people you like in that game you like, and then there's your addictive game!