Left 4 Dead 2 is the game to get a group of friends together to hop on with. A truly timeless masterpiece that just doesn't want to stop giving. With both the Left 4 Dead 1 and 2 campaigns accessible in L4D2, it's the ultimate zombie hoard shooter. It's extremely replayable due to the implementation of the director. The director can alter your run by deciding which items you'll find in specific locations, their frequency and quantity, and even sometimes altering map geometry. This feature alone makes the game almost infinitely replayable out the box. However, when you take into account that this game features the holy steam workshop, it takes things to completely new heights.

My friends and I have almost exclusively played this game with an absolute metric ton of mods as of recently, much to our frequent amusement. This, however, is in no way necessary to enjoy the game for hours upon hours with a group of friends.