I think GTA4 may well blow both of them out of the water, but Crackdown is really a very different game. You're a genetically-enhanced super soldier, and despite how lame and cliche that sounds, it allows you to do some really fun things you could never do in a GTA game, like jump a hundred feet from rooftop to rooftop, scale a skyscraper, pick up cars and throw them like toys, and so on.

And I'm in the Beta.