Skylanders: Spyro's Adventure was the best selling game over all platforms from January to April 2012, if one factors in the sales of all the figurines that go with the game.
Skylanders: Spyro's Adventure was released back in October, and had a unique mechanic in which players purchased figurines that sat on the base of a castle controller/device that would unlock the character it was based on, which would have all of its own special abilities. The figurines elevated to a "gotta catch them all", with rare characters being highly sought after, including special gold versions of the figurines.
Activision also announced that the game's sequel, Skylanders Giants, will be shown off at E3 in the next few days. It will add, of course, giant characters to play with.