I have TF2 installed on my other computer, but it crashed so I'm not planning on wasting 8 hours just to download it, and i barely played it.
I heard Dead Island and Left 4 Dead were amazing, I'll have to check those 3 out. I was also looking at Bioshock as well, since i heard it was pretty good, but it doesn't seem that interesting. I've been hearing EVERYONE talk about CSS, and i think i might get it. :D
Also, i heard HL1 was good as well.