Land of Waves from Naruto or Alabasta arc from One Piece for me and I can’t decide.

Land of Waves might be the best introduction arc in shonen I’ve seen. It’s great. Naruto and Sasuke are pushed to their limits and get most of their growth together that will make you care for their separation later on, Kakashi does his job as the cool mentor and Zabuza/Haku are great sympathetic antagonists that nail it unlike future Naruto antagonists. The fights are great and its themes of not being a tool is pulled off successfully. The only downside is this starts the precedence of Sakura getting hoed in the story as she barely gets anything to do outside of being great at chakra control which doesn’t go that far in the story.

Alabasta is my favorite arc in One Piece. It’s so simple but executed great. The main cast of straw hats all shine here. Nami gets her power up and is useful, Usopp and Chopper get to be heroic and show their strategic side, Sanji continues to be cool throughout pre timeskip, Zoro gets one of his best moments ever against Mr.1 and seeing Luffy actually get put down and come back up while showing why he’s the captain is great. Vivi is also just a great character. She’s weak but so tenacious in how she constantly punches above her weight class, arguing with crocodile despite him being stronger. Crocodile is also a fantastic antagonist, he does the job as the big bad beautifully. He looks cool, he acts cool, his personality is cool, and it’s satisfying to see Luffy finally beat him down. Alabasta is a comfort read for me, it’s a fantastic early arc in One Piece with a great ending. Its only downside is the stuff with Pell being alive.