Thanks for the invitation! I used a C64 as my only computer in the late 1990s long past its prime, because my mother got a really good deal on a whole set with printer and disk drives and plenty of disks with software (mostly games, from magazines). However, I was still a bit annoyed by the limitations of the system. I guess, if I had had a forth disk, I might have felt different.
In any case, for personal reasons I don't want to explore the C64 more.
But I never had a GameBoy Advance nor a Wonderswan.
Or the TurboGrafx-16 / PC Engine, Nintendo Entertainment System (alt), Commodore 64, Vic-20, Atari 2600, Atari 7800, Apple II/IIe, or Pet 2001 https://github.com/cc65/cc65
Or the ZX Spectrum, TRS-80, Apple II (alt), Gameboy (alt), Sega Master System (alt), and Game Gear (alt) https://github.com/z88dk/z88dk
Note: Some are slightly pre-1999 (all these, I have at least successfully made a "Hello World" with)
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If they're really wanting 1999, that's the 5th to 6th generation console range with Sega Saturn, PlayStation, Nintendo 64, and Dreamcast. (on these, only recommendations, no successful compiled software)
I wonder how the latest and greatest Wonderswan is doing in terms of price.