ZX-80, actually. Programmed that thing to the limits, which were extremely narrow. Learned a lot trying to cram program, OS, and video into just 1024 bytes. Even built a sound card for it, playing music the very hard way.
That was actually my THIRD home computer, following COSMAC Elf in 1976 and a KIM-1 a couple of years later.
Sold the KIM on eBay for over $2k a couple of years ago. Although I've divested myself of a lot of other personal computer stuff in recent years, I certainly lucked out in picking the one that I thought might be worth a little $ someday!