If we're not counting games that were ported to other platforms then I gotta go with Hover!, which was a capture the flag game that came bundled with Windows 95. I know it was still compatible with Windows 10 but I'm not sure about Windows 11.
For nostalgia's sake, I'd have to go with The Sims. I spent hours on that game as a child, and I always had loads of fun. Haven't been as invested as an adult, but I enjoy picking it up every now and again for nostalgia's sake.
Stardew Valley would be up there too, but I know that's not PC exclusive anymore. I've also spent countless hours on League of Legends with friends and on my own over the years I played consistently, and I still go back to it on occasion when I have an itch to play.