Gohan vs Uub

Full potential.

Who has the higher ceiling? (Imagine what the old Kaio could do for Uub..)

Oob.

...But it obviously can't be 'proven' right now. Just a feeling.

a higher ceiling, I'd say Gohan because it's always been his thing. Vegeta and Goku have both said he's got a lot more potential than even they do if he'd train, and would be the strongest fighter.