Hard to say given they're all such different types of games, but I'd go with Viva Pinata purely on the basis that it's a really original and vibrant game.
I voted Rainbow Six AND Viva Pinata. Rainbow Six has a strong multiplayer component, whereas Viva Pinata is one of the best single-player experiences I've had in a while. Put it this way, do you want to shoot dudes in the face, or do you want to bribe weird animals into having sex by feeding them apples...?
Rainbow 6 is awesome! So is Gears of War (xbox live is great for both games)
Viva Pinata will keep you busy and oddly addicted for a loooooooooooooong time. If you are not a live gold member, Viva is the way to go.
If you are a live member and really like tactical FPS than Rainbow 6. Gears of war isn't overly tactical but a great game that keeps you very immersed.