Killing Floor [Steam store page]
The graphics are a bit rough but this game is all about speed and accuracy killing waves of experimental creatures. Forget story, the co-op is loads of fun with a group of friends, and fairly challenging as well.

The most difficult part is the way enemies rubberband - if you run full tilt on many maps, and turn around they'll still be very close behind you - rounding the nearest corner or doorway. The spawn locations also don't seem to be set specifically - they seem to appear directly around any corner that you haven't looked around in the last few seconds, or further ahead if you're running rather than camping. On some areas in maps they also drop down from above, off buildings or through holes in the roof.

These mechanics aren't necessarily flaws - they make the game much more fast paced and difficult than it would be if you could kite them more strategically. Kiting is still very much possible because the game limits the number of enemies on the map at any time - so if you can group them all behind you then they will slowly appear in front of you as you kill ones behind.

Team Fortress 2 [Steam store page]
I wanted to love this game, but found it hard to get into because it requires team effort (mainly a healer at your back) or you die exceptionally fast. Unlike Unreal Tournament or Counter-Strike: Source where you can easily play a quick lone wolf match