I don't wanna repeat what's already been posted, but I agree with the Hunter x Hunter fights posted so far. I'd just like to add this one though:
https://www.youtube.com/watch?v=RRKLB0vGeRE (Gon vs. Hisoka)
Past that, Bleach has a lot of fights that I've rewatched a million times. Here a few of them:
https://www.youtube.com/watch?v=b_qPYr2hrSQ (Kenpachi vs. Nnoitra)
https://www.youtube.com/watch?v=QNZWEBBXD7A (Grimmjow vs. Ichigo)
Not really a fight...but this pleased me: https://www.youtube.com/watch?v=trz0SIaU-5Q (Grimmjow vs. Useless Arrancar chicks)
I don't watch the One Piece anime, but from the manga, some of my favorite fights included:
Luffy vs. Lucci
Strawhats vs. Franky House
Whitebeard vs. Everyone
Luffy vs. Bellamy
NNID: Zephyr25 / PSN: Zephyr--25 / Switch: SW-4450-3680-7334
