Halo: CE
Halo 2
CoD4: MW
Halo: Reach
Gears of War
Mass Effect 1&2
Crysis
Fallout 3
Fallout NV (Haven't played personally)
S.T.A.L.K.E.R (Same as above)
Brink
Uncharted 1 (You've played 2, go play 1, as some other suggestions though, I've not played it.)
(Seriously why so many sequels but not the games before them?)
Portal
Portal 2
Battlefield (entire current Gen Series is supposed to be good but I haven't played them)
Metro 2033
Bioshock
Singularity
Borderlands
Dead Space
Dead Space 2 (am yet to play)
F.E.A.R. (I hear the series is good, I've not played them)
F.E.A.R. 2 (See above)
F.3.A.R. (See above, above)
If you have a friend with whom you can play local co-op:
Army of T.W.O.
Army of T.W.O. the 40[sup]th[/sup] Day
Left 4 Dead
Left 4 Dead 2 (unless you live in Australia, in which case keep boycotting)