Commodore 128

Three computers in one box: a C128 mode with 128K and 80 columns, a hardware C64 mode that is a real 6510 rather than an emulation, and a Z80 for CP/M. Bill Herd's team built it in five months, and it is the most technically ambitious 8-bit Commodore ever shipped. The C64 compatibility is what people actually used, which is both its strength and the reason its own mode has little software.