16-bit, especially if you include Neo-Geo. That era was truly the pinnacle of 2d gaming, and the console wars took it to new heights. Just look at games like Sonic, Street Fighter II, Ecco the Dolphin, Crono Trigger, and many others. The 16-bit era was also when developers really tried to push the boundaries of the hardware as much as possible (including 8-bit hardware). 8-bit series were also brought back and pushed to new levels, like with Dracula X on the PCE or Final Fantasy 6 on the SFC. Arcade ports that were close to the real thing also became a real possibility (in the early 16-bit years, anyway).
To this day, I play my 16-bit games far more than any other generation. I play a lot of 8-bit games also, but only those with truly outstanding gameplay. And as for the 32-bit systems, there are a lot of good games, but also a higher ratio of crap.