Metal Gear Solid 4. Plain and simple.
It surprised me, but the Xbox 360 wound up being a very "Playstation-like" experience. The main reason I got a 360 was because the PS3 was sold out. After I got my 360, I didn't really see a need for a PS3......then, MGS4 came out. I had to have it on day one and the only place to get it was on the PS3.
The rest is history.