I would say PC only games mainly consists of complex strategy games like StarCraft or Total War and small indie titles that were not ported to a console like Dwarf Fortress. I would argue in general that there aren't really any must play titles, it all depends on what genres you like.
I would recommend Oneshot, which is in the second category and similar to Undertale. While there is a console port, it loses a lot of appeal due to a certain spoilery mechanic if you play it not on a PC.
Other games I like in the indie category:
- FTL: Faster than Light
- Into the Breach
- Baba is you
- Renowned Explorers
On the strategy side there are real time strategy games like StarCraft 2, Age of Empires 4/Age of Mythology, all of the MOBA games like Dota 2 or complex 4X games like Hearts of Iron 1-4 and all of the Total War series etc. I personally like AoE4 and the new remake for AoM.
If you like those totally depends on you.
Except Half-Life, that one is PC only and must play. I suggest to play the remake Blackmesa and Half Life 2, then you don't need to play a 20 year old game.