My current favorite Android game is Marvel: War of Heroes. 

It's a mix between a mission focused game (think Mafia Wars, Vampires Live, etc) and a card collecting game (think Elements), that takes place in the Marvel world.

You complete missions to get cards, battle to get resources, and build the ultimate deck through a mixture of Boosting, Fusing and Mastery.  I can usually always find something to do when I log in, especially when they run special events, which is practically every week! 

My favorite aspect is the card collecting.  I like that you still get to upgrade the card and there's all sorts of methods that you can apply and all help you meet different ends. 

What are you waiting for?  Play today!  (and if you do, you should use my referral code so we can both get a special edition Jean Grey card!!!  My referral code is xyp397688)