I recommend CoD4, but I didn't play anymore after that.
Halo wise, I'd recommend all of them, although I have nostalgia filters on 1. Halo: Reach is actually a great start, since it takes place before 1. ODST can be safely skipped.
L4D2 is also great if you're okay with playing online. Same goes for TF2.
Bioshock is amazing. I never played 2.
Other than that, I'm drawing blanks.
--
I may laugh at my own jokes, but I don't dare laugh at SuperNiceDog!