For singleplayer only, Rage and Darksiders are the best value. Never played RUSE, but it's sort of like C&C, I'm assuming. RTS games are generally strongest in their multiplayer. If you can only buy one, I'd probably go with Darksiders, but Rage is a really good deal at $20.
rage and crysis 2 have pretty weak sp. crysis 2 is too easy and rage starts strong, but ends on a whimper far too soon. darksiders was a nice surprise. ruse i can't comment on, but if you haven't played metro 2033; maybe think about that. great game.
hat'ern. a person that calls you out on your douchebaggery