For quite some time World of Warcraft has been the most popular, but this year i feel like that could easily change.
Next week Age of Conan is coming out, a game with Mature content - True barbaric action, blood flowing and hacking and slashing at the ennemies .Its probbably not going to beat Wow in numbers, but can it beat it in true gameplay and graphics.
Later this year we may also see Warhammer Online. a quite popular, both in games and outside. A game that really seems to focus alot on PVP, more similar to Wow but also is something completly different.
To combat this another expansion to Wow: Wrath of the Lich King. Which i suppose is going to bring in some of the frozen throne topics, new area to explore, higher level cap. bascicly more stuff to do, but the game stays the same.
Its a good year for MMORGs, if you like Wow or if you are looking for someting different.