Left 4 Dead 2

Left 4 Dead is not to old yet to be forgotten and Left 4 Dead 2 is most certainly a worthy successor although its not to youthful either. It has the improved AI director which adjusts the game to the players to keep it challenging and interesting which also alters the number and variety of zombies depending on how you played.
Left 4 Dead 2 still hinges on the Co-Op portion and so by default you will usually play with other players online of course there are AIs still around for your friends in this horror game but in the end a human player just acts more interesting.
The game of course offers the Source Engine again which means it is in the end on par with all the other Source Engine powered games graphically none of which are to old to look nice yet but it does have a slightly aged look nowadays.

Left 4 Dead 2 builds on the same level staging as it predecessor did and this offers a lot of variety again offering levels to the player in which the player is trying to escape from the hordes to an aircraft or some other 'safe' location. Of course we are greeted with the same familiar weapon selection. Nothing especially not worthy here to be honest.

Overall if you are looking for a good zombie survival shooter with decent graphics and fun maps as well as replay-ability then Left 4 Dead 2 has definitely something to offer to you.