Of the systems you mention, I would go with the SNES thought the Genesis would be real close. The 16-bit era really represents the peak of 2D game development and there a lot of great games that just work better that way in my opinion. I vastly prefer 2D platformers to 3D platformers for instance. Of course, more modern machines are capable of spectacular 2D games but that's not where the best development efforts are placed.

But best system ever? The Commodore 64 without a doubt. :)