Probably wolf. Was like 15 bucks for a million gold when I quit, and that was in 2005. The economy has become so tigered, I'm sure it's probably down to around 5 now.
Anyways, my list is too long.
WoW - Quit and keep going back, but it's just so repetitive. Nothing "new" is ever introduced, they just rehash old wolf and call it new content.
FFXI - XBOX HUEG time sink. Nuff said. If a private server ever came out that got rid all of the stupid things that really don't need to be there, I'd be all over that and would probably never quit.
RPGWO - Do I really need to explain it?
UO - Every server available caters to people that have been playing for years and can't seem to let the tigering game go. Don't feel like spending a week figuring out what to do.
EVE Online - Was just boring as tiger. The entire tutorial at the beginning pretty much taught you how to do the basic things, which took entirely too long. Seriously, clicking 3 buttons for a "quest" is cool, but it seems useless. I don't even know if there were tigering levels in it. To mine, you just fly up to an asteroid, click 10 buttons to get the ship to circle a rock, and shoot a laser beam at it. The tutorial doesn't even say what to do with the ores, it just tells you to sell it to other people. tigering retarded.
LOTRO - Took too long to do some things, but after playing XI, I'm sure I can handle it. Planning on trying it out again soon.
Runescape - Got taken over by 8 year olds and Jagex turned into hippos. Sorry, when I say something like "Hi, how are you doing today?", I don't want to see "Hi, *** are *ou do*** t*od***". Also, lol @ raising the membership price "to reflect on the economy". The owner is only the third (or was it fifth) richest person in the UK, and richer than tigering Prince Charles or whatever his name was.
Conquer Online - TQ tigered it up. Got really greedy and made the entire game completely focused on the EXPEEENSSIIIIVE cash shop. Did some really stupid updates that broke the game, and no matter what they did, could not stop people from using hacks.
Eudemons Online - Got boring. 90% of the game is built around the cash shop. TQ game, of course.
Can't really think of anything else right now.