MGS4 only because I've played 1 and 2, on the pc and xbox (respectively), the others I could care less about, though the God Of War games are meant to be ok? (I've got a friend with a PS2)
FF games look like boring shite to me, blue dragon is more then enough of turn based strategy on the 360 for me (YAWN Zzzzzz).
If it had have been what game do I want next on the 360 my answer would be whatever Bungie are working on.
I had such a good time playing halo3.