Megadrive & AtariST have many classics that are already available on Amiga so DrMD and Castaway should definately sort you out there. GPEngine is also top notch on the gp32 for PCEngine games, 3 I can recommend off the top of my head are Raiden, Galaga90 and Liquid Kids.
The 8-Bit consoles also run a treat but to be honest I don't bother with SNES as it doesnt run as great as the AtariST and Megadrive on the gp32.
Sorry I can't think of more games at the moment have fun.
Edit: GP32 Spain have a nice top100 list of each system actually