Which bird thermals the best?

Turkey Vultures are excellent soaring birds, but have a very low wing
loading and use microlift near the ground, which is often patchy and
disorganized. They typically don't soar very high because they eat
carrion, which they locate by smell (Just Google it if it sounds too
unlikely). They are generally not much use to glider pilots for these
reasons.

Vultures that locate prey visually soar higher and appear to thermal
better. These include Africa's Cape Vulture and the American Black
Vulture, both of which I've found like to join gliders in thermals.
Most eagles and largish hawks soar very well, as do storks and
herons. Their L/D is actually worse than most modern sailplanes, but
their advantage lies in slower flying speeds that translates into
tighter turning circles and having a very sensitive vario.

Mike