Atom Zombie Smasher [Steam store page]
Fairly easy on default, yet nearly impossible to win without using the chooser or combatant modifier - since it often gives you a poor selection of mercenaries. The balance could use a bit of work, since chooser lets you faceroll - but overall fun.

Plants vs. Zombies GOTY Edition [Steam store page]
A fairly relaxing arcade game - while repetitive it can get tough on the endless levels, especially right before each of the endless achievements. Would be nice if the game remembered your choice of plants between levels

Infectonator : Survivors [Steam store page]
The game concept/design is solid - scavenge supplies, rebuild your base, try to rebuild the car to get out of town - but there are a couple fatal flaws that really hurt the gameplay.

The main one is the AI - it's dumb as a box of rocks, and will walk through toxic slime (which kills the character in a few seconds) without hesitation, even when there's an easy path around it. Most of my damage and deaths in this game were due to the AI making stupid decisions.

And when a character "dies", they can be revived for around 15 seconds - which is fine except that it takes like 7 seconds to actually revive them. The timer doesn't stop as you're reviving, which means that if you get to the character with 3 seconds left on their timer, there's no chance you can save them.

It makes it frustrating if you're in the middle of fighting zombies and can't start healing immediately. I'm sure there are upgrades to make heals faster, but it's still a flawed design.

Corporate Lifestyle Simulator [Steam store page]
This game was originally named and bundled as "Zombies." before being renamed and released on Steam.

I wasn't a big fan of the melee combat, but once you get shotguns and SMGs it makes killing the management a much more pleasant experience.

Space Pirates and Zombies [Steam store page]
Slow but solid gameplay, though there are some major balance/difficulty issues - and to counteract those you need to grind quite a bit. The graphics are similar to a space RTS, though you have far less units than a normal RTS.

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.

Left 4 Dead 2 [Steam store page]
Even though Killing Floor doesn't have a story line I still prefer it for co-op fun. This is incredibly easy, and the scripted way the special zombies pop out next to you is annoying unless you have the maps memorized.

This feels like a console game - smoothed movement, unchangable (close up) third person perspective. The combat is also poor and frustrating - with guns that seem like they were meant to be used with auto-aim. Melee combat is completely useless.