In 2008, Left 4 Dead 2 came out, completely blew my mind. Easily just as good as Left 4 Dead, although I much prefer the atmosphere of the first. I started playing it again recently, tremendous fun, especially when you're drunk or under the influence of some recreational drug.
The music in particular I'd say was the key element I just love so god damn much.
This and L4D1, probably two of my all time favourite games <333