To answer only one Anime character, I can clearly say it must be Saitama.

Saitama has proven he can save and conquer the world. With his super ability and fast senses. Every opponent he had has been so much easy for him. He has proven to the toughest opponents he faced that he literally does them so easy.

It was mentioned also that the last opponent of him was one of the conquerors of universe. The fact that he made it easy to defeat is already an attestation that he's the strongest anime character of all time.