Is there a world that you DREAD having to play through each time you go through KH1?
I'd choose Colloseum for its all out difficulty in some of the optional bosses. Yes, they're optional. But not to us completionists. That and I hate the fact that Hercules (one of my favorite Disney films) doesn't have a fleshed out world like the others. At least they kinda sorta remedied that in KH2.
For sheer tedium, I've got to go Tarzan's Deep Jungle. I'd forgotten just how boring that stage is. Go to camp. Go to treehouse. Go to camp. Go to treehouse. Go to camp. Go to treehouse. ENOUGH. Let me just beat the hell out of that shotty wielding noob Clayton and go to a planet that doesn't suck.