World of Warcraft.
Because it consumes and destroys free time like I consume Cheeseburgers... And I love me some cheeseburgers. Because if you want to have the best stuff you have to spend hours and hours grinding yourself silly to the point where the game isn't even fun any more and you find yourself doing it more out of habit than anything else. What makes it worse is the fact that if you want to continue playing, you have to keep forking over your hard-earned cash to Blizzard. And if that wasn't enough, every so often (If the trend is to be believed thus far, about once a year or so) Blizzard will release an expansion pack in an appallingly whoreish attempt to create more money out of less value. They will ask around $50 for it and not even give you a free month of playtime for your contribution to their gigantic pile of money.
I have never had a game, at least from my standpoint, devalue as quickly as WoW. Portal was 2 hours long, and when I got to the end of it I found myself asking if that was it, but I still feel that Valve wasn't picking my pocket. Even though they were just bits of code they had laying around the office, they did manage to keep me VERY entertained for those 2 hours. WoW started off being fun, but that died at about level 15 where I realized that it was an MMO and that I wasn't going to get very far on my own and every one around me was a complete twat.
It's been said (mostly by me, in overly loud tones to anyone who will listen) that an MMO is only as good as it's player base. I am not going to pay $15 a month plus $50 once a year to have a 12 year old kid from East Bumblefuck, Wyoming tell me what a noob I am and then proceed to go on an hour long tear of Chuck Norris jokes. And if that is your idea of fun, then I would suggest going to seek professional help. I hear Dr. Jack Kevorkian can do wonders for problems like that.