Eiichiro Oda's One Piece redefined adventure anime forever with its outlandish cast of pirates and dangerous, yet wondrous world. These pirates don't rely on guns and swords to seize their treasure, as they often possess great demonic powers which were bestowed upon them when they gobbled up the notorious Devil Fruit.
A series this filled with superhuman characters routinely delivers unbelievable matchups that are an absolute joy to watch play out. Even if characters don't posses Devil Fruit powers, they're still probably amazingly talented fighters, and pretty much every character can throw down when they have to.
Luffy Vs. Katakuri
Luffy Vs. Lucci
Luffy Vs. Doflamingo
Zoro Vs. Mr. 1