The 68k is a total CISC beast. I wouldn't recommend trying to emulate it as something "simple."
If you want extreme in simple, do 6502. It only has a handful of registers and no more than 256 opcodes.
Of course, if you don't mind using someone else's emulator core to create your VM, you could use just about anything. You could also create your own VM machine code, but that'd require you to write the assembly back end for any C compiler you used, which is fairly nontrivial.