Wow! What a great quotation at times of political turmoil here in Brazil.

At its best is an idiom and it means of or at the highest standard of quality possible.

Example: You’ll love their new album; it’s simply jazz at its best.

If we have a sentence like this: At best, Doug will be only an hour late. This means in the best possible scenario.

Now: He was never at his best early in the morning. It means he was never performing as well or effectively as he is able to, early in the morning.