An online game that you would recommend?

I despise it with all of my heart, but Team Fortress 2.

hmm i'd say try l4d2? it's pretty fun or resident evil?

The Lord of the Rings Online.

Transformice?

Tera online [coming soon on steam]

Path of Exile? It's releasing on Steam in about a week or two.

Planetside 2, wonderful ;)

You're going to need a really powerful pc...but, look at this! :D

If you have some friends these games too and want to play together, I'd suggest L4D2, Sniper Elite: Nazi Zombie Army, TF2, Dungeon & Dragons: Online, Lord of the Rings: Online.

War Thunder, there will be tanks soon.

Duel of Champions, one of the few non P2W TCg out here.

Aion, Lineage 2, Tera, World of Warcraft, Rift... you have many possibilities. Question is what kind of game you are looking for.

I recently started playing Vindictus, not a bad game imo.

DayZ, Killing Floor, Mount & Blade: Warband, America's Army 2 (using AA25Assist) an old free game with still a good gameplay.

MMO wise, a lot of them have gone free to play so you can download and test them out. You seem to have played most free shooters already though. Could try War Thunder, free on steam Flight Sim/Combat and rumor is ground force or naval may release on Nov.

Starcraft 2 - best game in the world :)

Bloodline Champions? though i haven't played it in a while, so don't know about player state :P

How about this game?
Just kidding bro

Horror survivor online? The Secret World I'd say. If it's only sorvival, try Fallen Earth, a post-apocalyptic MMORPG...

The description you added, I haven't played the game myself, but isn't Killing Floor like that?