Despite being relatively young, the limitless pool of strength makes Meruem unstoppable. The Chimera Ant King is intelligent and proficient in battle. His ability to grow and adapt to ever-changing situations makes him the strongest villain in the history of anime.
It's difficult to gauge the strength of One Punch Man's villain due to Saitama's limitless powers, However, Boros remains the most robust villain Saitama faced.
Madara Was the most powerful member of the uchiha clan who grew stronger with time.
Aizen's strength comes from his intellect and utterly ruthless nature. Although he has superpowers, he is no slouch in hand-to hand combat and sword battle.
Freiza remains the strongest villain in the canonical Dragon ball anime.