Another vote for the Texas Instruments 99/4A.
It was my first computer, and eased me into every subject related to computing. Rock solid from day 1, and reliable to this day.
The memory layout is simple, yet has interesting "features". These days it's guts are well understood and can be fully manipulated.
AtariAge.com has an active forum.
New software and fantastic hardware upgrades are available these days (like the F18A VGA internal adapter).
It was the first true 16 bit processor (8 bit bus) available to mere mortals like us. Many great games and programs were made for it, or ported. Miner 2049er comes to mind... Think Manic Miner.
I feel that Assembly Language programmers would find it an interesting counterpoint to the Speccy.
I'm interested in an FPGA based version of the Texas Instruments 99/4A computer system. A smart fellow, Erik Piehl/Speccery, in the AtariAge Forum (Project is in Hackaday.io) is working on a TMS9900 CPU Core FPGA.
https://hackaday.io/project/20826-tms99 ... re-in-vhdl