MGS4 is pretty good. If you're into any of the first 3 you should enjoy this one.
Depending on the genre you like here's some top games I suggest:
Platformers: Ratchet and Clank Future Tools of Destruction, Uncharted Drake's Fortuen
Shooters: Call of Duty 4 Modern Warfare, Resistance 1&2
RPG Adventure: Elder Scroll's Oblivion
Fighting: Street Fighter 4, Soul Calibur 4
Sports: MLB the Show, NBA 2K series, Skate
Some of these aren't PS3 exclusives and some have sequels coming out soon this fall. Personally, I'm giong to pick up Uncharted 2, CoD Modern Warfare 2, and God of War 3 when they come out. I'll also be picking up Batman Arkham Asylum when it drops in price (it just came out).