Doflamingo for me like some have stated already. He just fits so seamlessly into the world Oda constructed. His DF abilities, his ambitions, his role as an underworld broker, his limitations as a power house vs the actual top as well as a complacent power sitting on a unearned throne, his back story, his connections, his finessing the world to be protected by and essential to super powers of the world, his design, his intelligence (Even his lack thereof at times lol) his speeches etc all just works in OP. The ripple effect of his defeat has truly shaken up the OP world on a scale even beyond Whitebeards death at MarineFord.
Haven't even dived deeper into the themes and references of his character to the broader themes of will and heritage and family / Controlled chaos or religious parallels and references to other works of fiction and actual history. Doflamingo was so well written by Oda.
Honorable mentions
Luffy
Zoro (Pre Time Skip, coming into his own as a leader and right hand, several sacrifices of his own ambition and pride for his captain and crew just greatness)
Ace (Tragic character with father issues, most perfect death in shonen history imo)
Fisher Tiger (As a black man his story just hits me that much harder then it likely would other wise)
Sanji (Character focus, gags hold him back)
Robin (Pre Skip and whenever she actually gets focus Pos skip is great)
Future Potential contenders
Blackbeard (high hopes)
Aokiji
Shanks