R2 and K2 for me, I doubt I would buy Gears 2 for the PS3 if the game would be ported. This because Gears 1 wasn't really my type of game (too macho and IMO lack of depth), but I can understand the reasons why other people would digg that game as well.
I think all 3 games take a very different stance with regard to the 3D shooter gerne (IMO also Warhawk is pretty unique). All have their specific strongpoints.