Which bird thermals the best?

Penguins!

Buzzards etc have a much lower wing loading than we do so they'll climb
in small, weak thermals.

With their high wingloading then penguins can't afford to stop for
anything but the best.

Rarely seen soaring, but that's 'cos their inter-thermal speeds are much
higher (Wing loading again), and there's not many other pilots flying in
Antarctica to see them.

KN