MSX (specifically MSX2)
This was basically the C64 of Japan, in terms of being a comparatively low cost all in one computer/games console that was still affordable (rich kids got PC88/98 series).
I ended up getting a refurbished Panasonic FS-A1F upgraded to function 99% as an MSX2+, and I was quite impressed by the machine. Lack of scrolling hurts, though the top developers seemed to be able to get around it. Great built-in music chip, and the FM-PAC makes it even better for those games that support it. You can even write disk images using a USB floppy on modern PCs, last time I checked.
It's got a very powerful flash cart solution, but it's also somewhat difficult or counter-intuitive to use. It supports disk images, but it's weird about how to use them, so I just wrote a ton of 3.5" disks and use those, with the flash cart just used for ROM carts.
My only exposure to this computer was through the many, many articles in Retro Gamer about it and its games. I honestly never got why, it just looked completely primitive.
Several years later, my wife got me a ZX Spectrum Vega for my birthday and as I went through the games, I realized how great the library is. It's extremely impressive what people were able to make on that hardware.
I've got the ZX Spectrum core on my MiST FPGA. It's an excellent core, but I really wanted "the real thing" and ended up ordering a hardware recreation called the OMNI DESKTOP 128HQ (still waiting for it to arrive).