Resistance 2 Co Op I would recommend. You can go Split Screen Online with up to 6 other Players online for 8 Play Co Op. Very solid.
LittleBigPlanet is really great as well.
Modern Warfare 2 Spec Ops Mode is pretty solid as well.
I know now is not the time but Black Ops and LittleBigPlanet 2 looks to be big improvements over their previous games.
With Black Ops you can go Split Screen with your friend and play against AI Bots which will be much better than MW2 because Split Scren Multiplayer in MW2 was just you and your friends and it gets boring fast.
ALso Gran Turismo 5 has Split Screen but not Co Op so later on in the year you could look into these games. And they all release in November!