I've been playing Killing Floor for years and it is without a doubt my favourite game here.
+ The combat and carnage is unparalleled: explosions, dismemberment, and headshots look and sound great
+ The class and perk system is incredibly robust: classes have unique roles, pros/cons, levelling is addictive
+ Huge variety of weapons and maps adds a ton of replayability
+ Best free seasonal updates of any game out there
Fair warning, this is a skill-based game and it is difficult. Newcomers should start on Normal difficulty to learn the ropes and have fun. Playing on Suicidal and Hell On Earth requires a lot of experience, working as a team, and in-depth knowledge of game mechanics. For many people however - myself included - this is the real draw of the game. If you put in the time, playing on HoE with 5 other experienced players all working together is some of the most fun you can have online. There is also nothing more exhilarating than being the last one alive on Wave 10 knowing your entire team is watching and relying on you to stay alive.