Despite how well put together and fantastically made the Half Life series is, on top of its engaging story and characters, and despite how much I adore CS as one of the founding fathers of competitive FPS and its amazing gameplay.. I pick the Left 4 Dead series. There are just too many unforgettable moments with friends that happened in those games. Too much endless fun and too many laughs and memories.
TF2 was just never really my thing, and while I think the Portal games are great, I also think they really aren't in the same league as the others.
Would you like to know more? http://i.imgur.com/hIDW3.jpg