The Doom 1.0 demo, at the time, fitted on one and a half HD 3.5" floppy disks and was absolutely mindblowing. It really felt like taking a look into the future of virtual reality and showed what the peak technology of the time (silky smooth on a 486 DX2 with a PCI graphics card ... there was one at school I had access to, lol) was capable of!
Not a game, but the State of the Art demo for the Amiga was similarly mindblowing ... a full music video on one SH floppy disk :-)