World of Warcraft has the most players. Today we can add another title to the "Greater Than WoW" Club, as the official site displays a counter showing that Dota 2 has a larger playerbase.
Dota 2 had 7.86 million players log in during April, a number that outmatched World of Warcraft's current shrunken subscribership. Of course, it's important to keep in mind that we're comparing a MOBA to an MMO and a free-to-play title to a subscription game, but even so it's an impressive achievement for Valve.
Gamespot noted that League of Legends has both games eclipsed by a teeny, tiny amount -- something on the order of 67 million players per month.