This isn't the oldest game I've ever played but it is a really old one, and weirdly one that I've hardly ever heard of anyone else playing: The Colony. It was originally a Mac game, I believe, but I played it as a PC port. Heck, I still have the 5.25 disks, manual, and code wheel. It is one of the earliest examples of a FPS, and used wire frame for all the graphics. It was BattleZone shrunk into a FPS. 1988 from Mindscape. Anyone ever play this?
I also have original Pirates, Starflight, and Madden #1 for the PC, all on 5.25's. Pirates was fun, because you had to boot your PC with the Pirates disk to play. Starflight was unique because you *had* to make a copy of the game disk to play it. Starflight permanently modified the data files on the disk so if you wanted to play again later, you had to have a set of virgin disks.
I think I also have a copy of Sabotage for the Apple II on a 5.25. Notable only because used to use a hole punch to alter the disks so we could flip them over and use the back side to cram on more games.
Most important problem for me: why doesn't anyone make a USB 5.25 drive?