Commodore 64 to start with, but the oldest IBM clone type system that I have worked for any real length of time with was just a few years after that one and was an classic IBM XT
that ran DOS 1.0 off a floppy (might have been a HD in the 5-10 MB range) at a blazing 4.77 Megahertz or 8 MHZ in turbo mode.