The World's Finest Assassin Gets Reincarnated in Another World as an Aristocrat

The anime The World's Finest Assassin Gets Reincarnated in Another World as an Aristocrat (original title: Sekai Saikou no Ansatsusha, Isekai Kizoku ni Tensei suru) is a classic isekai fantasy series.

Quick synopsis: The world's greatest assassin, after a lifetime of serving as a tool for his organization and ultimately being betrayed and killed by them, encounters a goddess upon his death. She offers him reincarnation into a magical world of swords and sorcery, but with a mission: prevent the world's destruction by assassinating the prophesied Hero, who is fated to go out of control and bring about catastrophe after defeating the Demon Lord.

He accepts and is reborn as Lugh Tuatha Dé, the son of a noble family of assassins in the Alvan Kingdom. Retaining his memories and expertise, Lugh combines modern assassination techniques, knowledge from Earth, and the fantasy world's magic and secret clan skills to grow into an unparalleled killer. He builds a team (often including loyal and capable female allies), hones his abilities from childhood, and prepares for the eventual target while navigating noble life, training, and threats.

It's a mix of strategic assassination planning, skill-building, action, some harem elements, and a more tactical/competent take on the isekai formula rather than pure overpowered wish-fulfillment.