Look at portal and portal 2, the games were brilliant. TF2 when it first came out was a shining beacon of how to do a team based FPS. Half life 2 and its expacs were all brilliant and using in game puzzles was what I wanted. LFD was again a game I got a lot of enjoyment out of. LFD2 I think was the only game of theirs I didn't rate as much. Mainly because it was released within a year or so of LFD. I still enjoyed it though.