Like most kids these days, my son loved the Bakugan Battle Brawlers. The Japanese tv shot centers around kids from around the world playing against each others a game of cards and magnetic spheres that can transform themselves into powerful warriors. The concept is easy and remiscent of Pokemon but the difference with Bakugan is that the characters are actually interesting, fun and colourful.
In the last couple of months my son received way too many Bakugans. But it was last Christmas that Santa really surpassed himself and gave him tons of new Bakugans, card, handlers, gun and an actual Bakugan battle ground. Sega and Spin Master have created a very cool spring-loaded miniature figures engine, which open itself whenever it rolls into a special metal card.
I'd say that his favorite character is without a doubt the evil Masquerade.