Mine is definitely KHII, because it was the first one I ever saw. I remember watching it round my friends house when I wasn't in to KH and I instantly fell in love.
I love the Young Xehanort Design, I also love them all. I don't know what it is about it though, I just really like him over all the others. Xehanort is a really awesome character altogether though.
I really disagree with you. Like many have said it will not effect the game in anyway. People who haven't played the previous title's, its them missing out and they wont understand it. But just because that it will be on the xbox one does that really make you think "Wow, this game is so bad because they released it on another console" when your playing it? NO. You will enjoy it for what it is, a KH game.