There are many other great titles though but I would recommend these:

Gearsof war, assassins creed, halo 3, forza 2, bioshock, lost planet, stranglehold

If you already own a ps3 i think you only should go for gears of war first, since its a xbox360/pc exclusive title. And buy assassins creed and stranglehold on ps3, its better, but thats my oppinion.