Kid Buu wins.
1. He is too fast to even be touched by Hulk and can simply remain airborne and use his long range attacks.
2. Kid Buu cannot be harmed physically nor be knocked unconscious and will simply regenerate at an alarming rate.
3. If Hulk decides to fight Kid Buu in close combat, he will not be able to harm him however Kid Buu's attacks will eventually overwhelm him.
4. He could BFR him however it won't be that simple.
5. Kid Buu was shown to possess instant transmission and can use that to his advantage or teleport Hulk to space where he would eventually lose oxygen and die.
Kid Buu wins this battle.