Resident evil 2 is no doubt number 1, then dead island, then left 4 dead 2
Mine are
1st is call of duty world at war steam, xbox 360, playstation 3
2nd is killing floor steam
3rd is NO MORE ROOM IN HELL steam
1) Dead Rising
2) Resident Evil
3) Left 4 Dead
4) Every sequel to the above games
I gotta go with Dead Rising for the zombie purest in me, but the Resident Evil series wins for game immersion and the Left 4 Dead series wins for mutations and duel play as the zombies.