The KIM-1 had a 6502, the Elf had an 1802, and the ZX81 had a Z80 processor, all 8 bit processors which were approachable for beginners.
I really had a blast with that thing. Programing the interesting ISA was fun. It almost felt like it had 16 words of RAM and several K of registers, since all memory I/O had to go through the D register. The best part was that you could use any of the 16 GP regs as the PC, so you could do primitive task switching.
The simple circuit made it just as fun to expand its hardware capability (bump to 2K of RAM, eventually 8K CMOS battery backed RAM, KC tape interface, Hex keypad, a 20ma. loop interface to a barely working Model 33, and finally I scored an 1861 to add video out)
I later got a Timex/Sinclair 1000 as a gift. Not nearly as fun from a hardware perspective (though I eventually upgraded the the external 8K RAM cartridge to 16k by piggybacking additional chips, connecting the chip select pins to the the board with flying leads). However, the Z80 was another fun CPU to program for. Much more advanced memory addressing and the shadow register bank made for some fun hacks.
Never did own anything 6502 based, which kind of bums me out since the first computer I ever programmed was 6502 based (CBM PET 2001), but I was a ways off from figuring out assembly at the time.
I wanted a C64 or a VIC, but couldn't afford one. Oh well -- once I saved up enough to buy a used CoCo I was spoiled for anything else for years to come -- the 6809 was the peak of 8-bit CPU design, IMHO.