This would have been easier if there was a winner in The Lost Adventures :(
I'd say Toph. Bumi is shown to only know how to bend earth. Toph taight herself to metalbend and more recently sandbend.
Bumi did take back Omashu by himself but I think the ability to learn create another bending art takes more skill.

XD

she is blind and still she kicks ass

Bumi's been around FOREVER, and Toph is blind. Of course Toph.

Toph, but i think maybe Bumi is wiser

Duh!!
TOPH!!!!!!!!!!! x]

Well my guess is King Bumi.

Both are amazing.

Invented metalbending.