My contention is the following:
Earth: King Bumi and Toph.
I believe Toph has greater raw talent, but King Bumi has more experience. Therefor, Bumi would win in a fight. He would take advantage of her blindness, and make quicksand or something in order to prevent her from 'seeing'.
Airbending: Of course Aang.
Waterbending; Paku and Katara
Firebending: Iroh, Azula, Azulon, and Oizi.
Both Azula and Azulon are described as being firebending prodigies. It is clear they have the most raw talent. Iroh has the most experience. It is shown that he can easily defeat Azula, and most likely Oizi (his doubt is really just humility).
What are you thoughts?