Superman wouldn't stand a chance against Kid Buu, it's pretty simple.
* Buu can replicate any technique he's seen, after being shown just once. For example his ability to turn people to candy was something that he replicated after watching Bibidis magic. This goes to even high level techniques. In other words, the first time that Superman used heat vision, ever after that, Buu could use heat vision. Same goes for cold breath, etc..
* In the manga Kid Buu destroys the entire solar system with a blast that extends beyond the outer planets, with one shot.
* Superspeed and fighting ability, can learn fighting skills after watching them being used only once. So for example if Superman used T-Vo, Buu would also be able to use T-vo ever afterwards. So you have one of the best hand to hand fighters in the universe. Buu's speed far exceeds faster than light, and this is while fighting no less.
* Durability and flesh manipulation. Buu can stretch and move basically like Mr. Fantastic, can be blown into a hundred bits and reform himself, and his flesh is extra durable to blunt attacks. In other words Superman's punches will do little to him.
* Transmutation: Buu can transmute anybody up to and including high level beings into candy and eat them, fully gaining their abilities. There was no character that was able to resist this effect once being hit by it in DBZ. The Kaio's were basically the skyfather level beings of DBZ, the one he absorbed was Dai Kaioshin (God of the universe).
* Magic: Since Buu is basically a Demon, his attacks and techniques are formed from magic, which superman has a slight weakness to. Buu's transmutation attacks are on the highest order of magic attacks, so Buu will be able to absorb his powers no problem once he physically subdues him.
Basically Superman has no chance of winning here. The arguments in Superman vs. Goku fights don't apply with Superman vs. Kid Buu, namely the amping (time spent increasing energy level : Kid Buu starts off strong from the get-go, just like superman), physical punishment taking ability, and speed blitzing.