Personally, I'm a fan of both World of Warcraft and EVE. WoW lends itself nicely to my casual playstyle and hardcore gaming swings, though I got bored once I hit 70 due to lack of solo content (I like grouping with friends, but only Kelvin and I were 70 at the time in the guild). EVE also piques my interest, for the sheer fact that there is no structure to it, and I love doing math (No, really, I do!). If you want a traditional fantasy MMO, WoW definitely has its perks, though Everquest 2 may be a good alternative if you're a hardcore player. If you like Sci-fi and don't mind the metagame work you'll be doing for it, EVE is worth the subscription fee.