Besides Avatar Kyoshi? It would definitely be Bumi. People have been saying that Toph is the most powerful bender, but she isn't. Reasons being, she is a kid; she is weak in terms of physical appearance and moset importantly she is blind (even though this has never been a problem for her). Bumi is a more experienced bender and has the unique ability to bend with his eyes (something that not even Toph has mastered).