I would have to say pokemon because that's what I spent most of my time playing growing up. I love the others but pokemon rules them all!
Mario, for sure. He's had consistent, good games, most of which are higher rated than most games by those other figures. My second preference would be Pokemon, followed by Sonic. Unfortunately, I haven't actually played a Kirby game.
I did grow up playing sonic and mario - and mario is my fav. But when my boy was little he played pokemon - which was actually quite good fun!
When I was younger and had to make a choice between Nintendo or Sega, I picked Nintendo. So Mario wins for me.