The Wii is bloody excellent for emulation. Mine is chipped and connected to the projector. C64 and speccy are well supported ..... yet to get the Amiga emu doing anything great though.....
I have all the mentioned consoles running emulation and I think the Wii is definitely the best. The actual emulation is better than on the others but the Wiimote and Classic Controller also seem more suited to the old systems than the other controllers. Also N64 emulation with a Gamecube pad plays really well.
Having said all this, the best NES emulator I have is actually for the PS1 and is called It Might Be NES. The front end is quite basic but the emulation is strangely better than anything else, even than a PC. It is hard to explain why but it could easily pass for the real thing.
Another great emulation console is the hand held GP32 console. The batteries last for many hours and it is excellent at emulating just about every 8bit system there ever was. It also does a bit of 16 bit stuff which is fairly impressive on Mega Drive but not so great for SNES.