Sonic easily (although I don't count any of the 3D games as being part of the series)

Although that may be a bit biased because Sonic (for mastersystem) was my very first game and I only ever owned a NES. But even to this day I can go back and enjoy them as much as I did back then.

As for Mario, I just don't like it. After I went though my big Sonic revival stage I played Super Mario World for SNES and I just couldn't play through it. It was just seemed slow, clunky and a little too surreal for my tastes.