Definitely World of Warcraft, I have over two months in time played in that game, I just recently stopped playing, but I couldn't go without it for a while a few years ago.
I have been addicted to other games like TES:IV Oblivion, CS:S, TF2, and Fallout 3, but for much shorter lengths of time.
I regularly take breaks from games so I don't get addicted. I have a healthy spread of different games I play at once now.