Red Dead Redemption 1. Some people played the sequel without playing the first and refuse to until it gets a PC port.
This is not going to happen because the original production of RDR1 (the actual programming part at least) was an absolute shitshow, and the incredibly spaghettified code (another reason why a port isn't going to happen) was bounced around like crazy, and now no one knows where it actually is.
Rockstar might still have it on some old hard drive somewhere but considering how source code preservation has only just become a big deal that even major publishers are just starting to realize, and how long it's been since its original realse, there's a good chance it's lost forever.