i would say gun bros because there are missions and what seems like endless waves of enemies, which you can get money for then you go the gun bros store and you can buy powerups,guns,and armor, you can buy them with coins or warbucks, you can get warbucks by getting your daily bonus which lets you know on the front page.
JetPack Joyride! Game is insanely addicting. It might not be free anymore but it was when I got it. Since it's not free anymore, I'll rule it out. Temple Run, though, is insanely addicting!