Theoretically, the NES should be the system pushed most to its limits because it has had the most time to be developed for. If a modern programming team went back and made an NES game it would look like an SNES game from 18 years ago.
I believe this is also true in practice, as it is a lot easier to make sure that you have optimized a game that fits into a few hundred kilobytes than it is to check a game with thousands of times more data. Example: because of cartridge limitations non-pirate NES games were all less than around 1 megabyte, and wii disk size is in the gigabytes, It is much easier to push a NES game.
Trust me, I program graphical TI-BASIC games (yes, the ones that you all played during math class in high school) and you only get 20 kilobytes to work with. You'd be surprised what you can do with a 6 megahertz processor, a 95x63 black and white screen, and 20 kilobytes.
So I think that NES with Kirby's Adventure wins.
I'm not a fanboy, I just don't enjoy dual analog control. It's d-pad or wii-mote for me.
the conduit has changed the way wii play games. 
I know. I'm sick of the puns too.
