I've tried several, but WoW is my personal favorite, even though I became so addicted that I stopped playing last summer lol Several reasons why, number 1, there doesn't seem to be as many "kids" on it as there is on Runescape, besides that, I love that you have the option to do things on your own (quests, dailies, etc) or you can group up with friends, guildies, w/e for doing instances, raids, etc. But I could be a little biased since I had real life friends playing, that helps a lot.
However, I am checking out Aion when it comes out later this year...I mean come on, you have wings
