They all have their place. I'm going to have to go with Mario. I've enjoyed all of those series before, but Mario is the series that sticks. Got addicted to super mario land on black and white gameboy and never looked back.
Definately Sonic. Sonic 1 was the first game I ever played and the 2D sonic games for the sega are amazing and all part of my childhood. Mario is awesome too though!
I grew up on Mario so it's Mario for me. The 3D games are downgrade for me though. Pokemon as a franchise (show, cards, games, etc.) could have been my favorite if they had stopped adding Pokemon after the 2nd generation. I mean keep the show going, expand gameplay mechanics, keep releasing new cards but without adding new Pokemon. The quality of the games has been going up but the quality of the actual Pokemon is falling.