(In no specific order.)

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Half-Life series (including Black Mesa )
Every Half-Life game is great (maybe not Blue Shift). I also like Black Mesa (a fan remake of HL1).

Minecraft Dungeons
Beat the base game and every DLC on Xbox. I preordered the 2nd one on Steam.

Detroit: Become Human
This game is amazing. Every little choice matters and can lead to your death or the death of others. Your dialogue matters, it affects how other characters think of you; they may even kill you in one playthrough but be your best friend in another depending on how you act!

GTA 5 Online
I've played GTA 5 online on console for a while, but I recently got in on Steam. I don't really like story mode in GTA 5, so that's why I'm mentioning Online in specific.

Halo games
I played many of the Halo games on Xbox. I may also get them on Steam.

Marvel's Spider-Man + Marvel's Spider-Man 2
Played the original on PS4, then replayed the series on Steam.

Goat Simulator (and the sequels)
Played a ton on Xbox and recently picked up the whole series on Steam.

Superhot (and the other 2 superhot games)
I used to play Superhot so much on my Xbox, and I now have both flatscreen titles on Steam and Superhot VR on Oculus (I also want the VR version on Steam so I can make mods for it).

BattleBlock Theater
Mostly played on Xbox, but I also own it on Steam.

Pikuniku
Another favorite I played a lot on Xbox and now have it on Steam.

Gang Beasts
Always a fun game with friends. Also played on Xbox.

Totally Accurate Battle Simulator (TABS)
Great for messing around. I've even made mods for it on Thunderstore.

Garry's Mod
One of those games I can always jump into just to experiment and mess around.

Undertale
dog.

Slime Rancher
Played it a lot on Xbox, but I still love it on Steam.

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Most of these games were originally played on console before I bought them on Steam, so my playtime here doesn't always reflect how much I've actually played.