If I think of 'PC' as a platform, the ones I loved that also help represent the platform itself are:
DOOM
Diablo 2
Command & Conquer
They've all been ported eventually (except for Diablo 2 which remained on PC for the longest time, until just recently with Resurrected). But those three games made the majority of their contribution to make me a "PC Gamer" since around 2004 (when I got my first 'real', self-made gaming-capable PC). I first played DOOM at a friend's place on his 486 (and played LAN, too), even though I was a console gamer only back then (but it blew my mind, obviously). I played C&C at a friend's place too (not the same person) who owned a good PC, played that and also Red Alert; as soon as I got my PlayStation in the summer of 1997 (after getting my first ever job during summer school vacation) I saved money to buy Command & Conquer that way.
Fast forward to 2004, I get myself my first PC (bought my own components, built it myself) and the first game I buy for it is Diablo 2 (and Lord of Destruction). Not too long after that, got C&C1, C&C2 (Tiberian Sun) and Red Alert 2. I eventually also get Ultimate DOOM, and DOOM 2.
So yeah, DOOM, C&C and Diablo 2 have always been my 'Trifecta' of PC games. To this very day. I still play DOOM regularly enough (from the base DOS versions, to all the variations you can think of, along with custom community WADs, or various 3D engines to run them with, or DOOM 64 EX, to the recent DOOM with Ray Tracing, any one of them), I made a decent 'come back' to Diablo 2 with Resurrected recently (played about a month) and I still play Command & Conquer mostly thanks to the great (can't believe I do say this but yes, it is great) Remaster they made for it and Red Alert. Franchises that never 'died', never became forgotten and still go on today so many years later. They're exceptional.