Ideas of good games?

System Shock 2

One of the best games ever made. Absolutely outstanding shooter/RPG hybrid with a great atmosphere and an amazing villain. You don't need to have played the first one to enjoy it.

Dust: An Elysian Tail

Beautiful metroidvania-style game with a slightly brawlery-feel to it. Play it!

Brothers- A Tale of Two Sons

Short, sweet, and does a great job at telling its story.

Vanguard Princess

I know that the game has been in about every single bundle, twice, and free more times than the average human can count to, but no need to list it twice ;)

Super Meat Boy, Binding of Isaac: Rebirth, and the Trine series to name a few.

My friend who likes games like this said Anodyne is the best game and the nicest surprise he has played in years so you might to want to check it out :)

Portal and Portal 2

At the first glace:
Undertale and Knights of Pen and Paper +1 seem to fit your criteria.
Brothers- A Tale of Two Sons, A Story About My Uncle, The Talos Principle are great though they don't fit your description.
Life Is Strange™ - if you have all the seasons, play, if not, it can wait.
Saints Row IV is worth a try if you like action and comedy.
The Old City: Leviathan is a walking simulator with great atmosphere.

If retro games and old style rpgs are your thing, I'd tell you to play some Final Fantasy games (especially IV and VI).These aren't on steam, but you can always emulate them. I really liked Call of Duty: WAW, it's my fav call of duty. Pid is a freebie, but it's cool. And if I were you, I'd definitely spend some time playing payday 2.

Retro you say ?
Original X-com: UFO defense, or Master of Orion 2 (when it goes on sale by itself) should be on the list if you like strategy games

Agarest: Generations of War, Breath of Death VII, Chantelise, Cthulu Saves the World, The Last Remnant, Recettear: An Item Shop's Tale, Ys series; the Tales series (i.e. Tales of Symphonia and Tales of Zestiria).
You have Vanguard Princess listed twice

Sakura Clicker :D
edit:
Counter-Strike Nexon: Zombies is total crap
Dead Island: Epidemic is dead, they switched it off

Definitely play this then come back and tell us how it was:
Brothers- A Tale of Two Sons

I know a very good RPG but it's for DS , or Atari if you prefer , it's called Front Mission and is about a war like the Cold War but in the future , the combat is in turns but it's very fun with interesting story

Dishonored if you haven't already.

I recommend that you play CoD: World at War.
I recommend that you purchase This War of Mine.

You have many really good games --

System Shock 2 (definitely)
Life is Strange
Knights of Pen and Paper (loved that game)
Faerie Solitaire (for relaxing between other games)
Left 4 Dead (it's good any time, really)
Story About My Uncle (fun & gorgeous game that I still need to finish -_- )
Dust: An Elysian Tale (another must-play imho)
I don't own Brothers, but I've heard nothing but good things about that one.

That's my two cents. :X

Borderlands and borderlands 2
Scribblenauts Unlimited
Terraria
Fallout 3 and New vegas

The Witcher 3, only $25

http://www.greenmangaming.com/s/us/en/pc/games/action/witcher-3-wild-hunt/

Game of the decade

This one is worth playing might wait for a good sale
it is a bit dated but the story is quite good and the combat is fantastic
personally have it for Dreamcast and also got it for PC ;o

Grandia 2

You should try all the games you own, no?

UNDERTALE. GO PLAY IT. I won it here about a week ago and I immediately fell in love with it :)