Runescape. Played it for hours every day. I even made a daily regiment of walking down to the library after school to play it, and went home at close to play a bit more. Woke up whenever the library opened on weekends and stayed most of the day or until close even to play. My home internet connection wasn't that great (dialup), but if I couldn't get to the library to play, it sufficed. Fun times :p
Then everything changed one day. I remember being suspended from school for some silly reason, and that same day I got a new laptop, and high-speed internet. And I also tried out WoW (vanilla) for the first time that day. Was pretty much addicted to that for a while, then I stopped playing most of TBC. Got back into it through most of WotLK and about half-way through Cata (halfway being in the time it launched to MoP).
From that point, I pretty much hopped to various other MMORPGs. Was really into Vindictus for a bit, but interest died off for some reason. I come back to it every now and then though. I played Guild Wars 2 for a good while, but can't find anyone who really plays it (tried hard to even convince friends to play to no avail). Tried going back to Runescape and couldn't (I do want to try out the old school version at some point though). I even setup a server for vanilla WoW, which although fun, can get semi-boring by yourself (perhaps I should just join a populated server or something).
I'm into Dota 2 mostly nowadays.