While this list focuses on the most popular, honorable mentions go to Fullmetal Alchemist: Brotherhood (the perfect anime), Vinland Saga (no enemies), Spy x Family (wholesome action-comedy), and Berserk (the best manga for those brave enough).
The emotional payoff. One Piece makes you laugh, then rips your heart out (see: Going Merry, Nico Robin's "I want to live"). While the length scares people, those who commit say the journey is worth every second.
Tatsuki Fujimoto is a madman. He breaks all shonen rules. Characters die randomly. The tone swings from slapstick comedy to arthouse horror in one panel. It is violent, horny, and deeply sad.