Sonic by a mile.
From the age of 3 to the age of 10, all I gave a fuck about was Sonic and Michael Jackson. I would, years later, find out the two were strangely linked, and not just because of the awesome Moonwalker game.
I still have my Sega Mega Drive (Genesis for you Americans) somewhere, but all the cables are broken. I have an emulator with all the classics and more, but one day I'll buy another Mega Drive just for the retro factor.