There are several anime characters known for their sadistic smiles, but one of the most notorious examples is Hisoka Morow from the anime and manga series "Hunter x Hunter." Hisoka is a skilled and manipulative character who takes pleasure in fighting strong opponents and has a penchant for sadistic behavior.
Hisoka's smile is often depicted as sinister and unsettling, especially when he is engaged in combat or when he finds amusement in other characters' struggles. His sadistic nature and love for cruelty make his smile stand out as one of the most chilling and memorable in anime.
It's worth noting that anime characters with sadistic smiles are often portrayed as antagonists or morally ambiguous individuals, and they serve to add tension and intensity to the story.