OK, so last week, I asked you all what your favorite retrocomputing (20+ year old) platforms are.
Looks like the ones that stand out (more than one person into them) are the classic Mac OS (three people), Palm OS (two people), and Amiga (two people) - there's some downright obscure stuff in here.
TBH I'll probably do the thing anyways, and just ignore what I found.
Which means it'd probably be on the (8-bit) Apple II or the (8088-based) IBM PC, the platforms that I'd say I'm most familiar with.
(Well, OK, I'm more familiar with later eras of IBM PC, but I'm currently more immersed in XT-era hardware. And if it runs on an XT and doesn't do something stupid or insanely awesome (see 8088 MPH), it'll run on basically anything newer.)