Most people will recommend the CHIP-8 system, because it is simple to handle - very simple graphics, very simple CPU, and a decent range of games to play with.

Most people will recommend the CHIP-8 system, because it is simple to handle - very simple graphics, very simple CPU, and a decent range of games to play with. If you're thinking "old school" then your choices are NES, Gameboy, and 8080. Personally I wrote an 8086 emulator, to play space-invaders. 8080 is a reasonably simple system, without horribly complicated graphics or MMU addons. Interrupts were hard to debug, but otherwise it's a very well documented system with lots of directions you can go in afterwards - emulate DOS, and Hercules? Emulate CP/M? Emulate other games? Or start looking into running Windows even!