You can give this run and gun a shot, best contra game I've played since contra 4
Destruction Paper
www.lincolnonaphoneproductions.com
https://www.youtube.com/watch?v=reV35YI4C60
the guy wants 2 bucks for it, I bought it, totally worth it

Civilization V (aka game of the forever)
Anno 1404 (play this - still very beautiful - classic from 2009 and ignore the vastly overrated Tropico franchise)
Cities: Skylines
Divinity: Original Sin Enhanced Edition
Kerbal Space Program
Shadowrun: Dragonfall Director's Cut
Astebreed
Homeworld Remastered
Europa Universalis IV
Darkest Dungeon
Sunless Sea
The Banner Saga
Age of Wonders III
Wasteland 2
Undertale
Stardew Valley
MirrorMoon EP
Endless Space
Endless Legend
War for the Overworld
Mark of the Ninja
The Swapper
Transistor
Hand of Fate
Armello

Best releases from the most underrated PC genre:
Monkey Island 1+2 SE / Day of the Tentacle Remastered / Grim Fandango Remastered / The Book of Unwritten Tales 1+2 / Deponia 1-4 / The Dark Eye: Chains of Satinav / Memoria / Her Story / To the Moon / The Journey Down / The Silent Age / Botanicula / Samorost 3 / Gemini Rue / Technobabylon / Shardlight / Primordia / Blackwell 1-5 / Heaven's Hope / The Order of the Thorne - The King's Challenge / J.U.L.I.A. - Among the Stars / Nelly Cootalot: The Fowl Fleet

Only PC exclusives, or can I reccomend Portal 2?

Also, Sanctum 2. It's also a multiplat, but chances are you never played it on consoles, and it's better on PC either way. It's a FPS/tower defense hybrid (I know, it sounds a bit crazy, but holy shit is it awesome. I've put ~150 hours into it). I'll link to a vid showing some gameplay so you'll get the idea

https://www.youtube.com/watch?v=IbUkZM7UM2Y
(Actually just watch that review, it explains it pretty well and it does a good job of showing off the gameplay. Also it's only 5 mins).