I know where everyone is coming from picking Roxas cuz he's a more seriouse character and there are some parts in KH I and II where I wish Sora was more like him, but I don't think Kingdom Hearts would work without Sora.
Even though he acts like a kid he has a strong sense of right and wrong that Roxas lacks, plus I thought the first few hours of KH2 were a little dragged out, I really couldn't wait to get to the part when Sora came in. So i would have to pick Sora, even though I wish sometimes wish he wasn't so goody goody. Hey maybe in KH3 Sora will be a bad a$$ lol