Had to be Son Goku from Dragonball. This guy is not just the strongest on earth, not the galaxy, not the universe, but the strongest in all 12 universes!! How crazy is that. Ok, maybe there are someone else who might be stronger than him like the great priest or Zemo. But Goku is the main protagonist of this anime and so he shall be regarded as the strongest anime character of all time.
Outside of Dragonball, the other newer character is probably Saitama from One Punch Man. Saitama is designed to be the strongest character in this anime and he had even defeated an alien who claimed to had reigned over many galaxies out there. In that battle, we were told that Saitama did not even try hard enough. His signature move is always a single punch that is capable of defeating any enemy.
There are many other anime characters who are strong in their own ways. But all of those anime are set on Earth and so, I would say Goku, who had defeated all opponents from 12 different universe, is the strongest of all time.
1. Koro Sensei
Koro Sensei is an anime that acts as the most dangerous hitman in the Assassination Classroom anime series, so sadistic that he was even dubbed the god of death. After he obtained the tentacle form, he finally gained extraordinary power that was beyond reason, even in his anime story he was able to run at an incredible speed reaching 24696 km / hour.
Thanks to that ability, Koro Sensei became a monster that was very difficult to defeat, even he was able to avoid bullets being fired at him very easily.
2. Marshall D. Teach
Marshal D. Teach is an anime character who plays the main antagonist role in the anime series One Piece, with the power of yami yami no mi, it has the ability to devour all the things around it.
Even after the great war that took place at Marineford, he gained the power of the strongest man at the time. Whitebeard who has the power of Gura Gura No Mi, he is able to shake the earth with the power of an earthquake that can even trigger a tsunami.
Armed with these two powerful demonic powers that made him very difficult to defeat, he was even able to absorb all the attacks and strengths of his enemies. So strong, he was also able to destroy all his enemies with an instant and even he was able to sink an island with the power of the earthquake he had.
3. Byakuran Gesso
Byakuran Gesso himself is a fighter who is also the head of the mafia in the series, but that is not what makes him very strong. His other strengths are Byakuran who is able to enter other parallel worlds and also uses technology from each of the parallel worlds he has visited before. The technology he obtained made him very difficult to defeat, but because of his character who acted as an antagonist.
Zeref is the strongest and invincible anime character. This is because, Zeref has extraordinary power and even he has the power of death that makes him unable to die. If Zeref is angry, then he can kill all living things around him.
This is a really subjective question as the inverses in which the anime story takes place can be totally different from each other. This means that the comparison of strength between the characters of the same anime makes mush more sense. Although I can list some really strong characters from anime.
I would gladly go for saitama of one punch man. This dude has so much super strength that he defeated an alien god without breaking a sweat.
There are also various anime characters who have proven to possess ultimate power of destruction.
Sage of six paths from Naruto
Son Goku from Dragon Ball
Admiral kaizoku, marshal d teach, Big mama all from one piece.
And many others I can't mention.
Personally I'd go for Saitama from one punch Man as the strongest hero of all time in the history of anime's.
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.