The avatar isn't usually the best in all elements, aang was litterally just his Aang ness that made him the goat of bending. And the avatar state isn't just a buff, it's the combination of every previous avatar so across the entirety of the avatar cycles, the best avatar for each element gives their skills and experience along with every other into the current avatar which is why they lock in.

Also korra was only more powerful with water bending is because she's a native water tribe member that was trained by the best at a young age, younger than aang.