World of Warcraft - Huge. Why? It's just Everquest, but better. Well, it is (was) the next generation of MMO. There was a gap between EQ and EQ2 and WoW jumped in there right when EQ aged and lost traction and EQ2 hadn't caught on (for a variety of reasons.)
There will be a time when WoW no longer holds up. Will WoW2 replace it, or will a new game by a new developer fill the gap? My guess is that Blizzard and every other developer who wants to release an MMO is working to identify that gap, and hit it with the best product they can. Blizzard's goal is to make that gap as short as possible, reducing competition's chance to wedge themselves in there. They've got a huge advantage...just like Sony and Everquest did.
I'm curious to see what supplants WoW. I'm glad I don't work for a company who's trying to shoot that gap (besides Blizzard, at least.)
Other sure fire hits: New Coke, Playstation 3, Everquest 2, and Godfather part III.
Most people are only going to pay $15 per month for one game. The best game. "Best", however, is defined by a lot of factors, many external to the game itself.