I'm gonna say Mario because they were always able to upgrade him and make better and better video games. From back in the NES days to SNES to 64 to Gamecube to Wii. Not to mention, back in the day, Mario 1, 2, and 3 were all completey different video games. Unlike Sonic where they were all the same and eventually got boring.
Mario's spin offs are a lot more fun too. Especially Mario Kart.