Team Fortress 2

I'd say every Valve game released after Counter Strike: Source is a great experience for PC gamers.

Forget all the mentioned above and play the three Age of Empires games.

Oh, right, if you've never played Portal 2, go get it immediately. Amazing game. Best if you play Portal first but just keep in mind that Portal 2 is superior in almost all ways.

As another treat, Valkyria Chronicles is now on PC. Easily one of the best PS3 games, it has a fantastic story and engaging tactical turn-based combat.

Civilization V, Team Fortress 2, XCOM: Enemy Unknown. also any Bethesda games and derivatives like New Vegas etc, even if you played them on console already you should play them again with mods.

You need classic??? = Black and White 1 & 2, Hospital what ever ( i forgot ) it's from EA, a very old games , Age of Empire Series, Total War Samurai, Civilization V & All Expansion Pack, etc so many LOl

I would get (and have):

Sid Meier's Pirates
Cities: Skylines
Grand Theft Auto 5
Halo: Combat Evolved

and eventually No Man's Sky

Total war Shogun 2, that is the pc game that left it's mark for me, before that I didn't have an interest in pc exclusive games.

2 Pages of PC games and no recommendation of Half Life or Half Life 2? Yall be cray cray! (Unless I missed it)

Oh and Command and Conquer games

Both Starcraft Games.
Both Halo Games.
Both Half-Life Games.
Both Star Wars: Knights of the Old Republic Games.
As you can see, I like Sci-Fi Games. (And games that come in two, obviously.)