The fight scenes in general are all great. The animation quality helps a lot, but the fights themselves are also well put together.
The opening fight scene between the Marines and Neo Marines. It starts with the lower Mooks from both parties fighting, set to a beautiful rendition of Ocean's Guide, sung by Z himself. The music then changes to a more intense tune as the higher members join the fight. Z himself one-shots two giants much bigger than him, on top of defeating several soldiers by himself. The final part of the fight is an awesome duel between Z and Kizaru, which ends with Z destroying the island they're fighting on with one of the Dyna Stones.
The final fight between the Straw Hats and Neo Marines is, simply put, epic. The lower troops drop like flies, even after the Monster Trio go on to battling Z, Ain and Bins. Franky even takes out the Shiro Kumas with his Franky Shogun, before finishing them off in proper fashion with a Radical Beam. Then come the duels between Ain and Zoro, and Sanji and Bins. Most of these two fights is shown in one long, awesome take, before the finishing moves, which are just as cool. The final part is a duel between Luffy and Z. Both fighters start off with strong attacks, then resort to more classic punches after they get tired enough. It may not be as spectacular as other fights, but it is much more believable, tense, and climactic. It boils down to a slugfest between the two fighters, during which you can feel how much each punch hurts.
And of course, there's Z's last stand. Z fights to the bitter end to allow the Straw Hats, Ain, Bins and Aokiji enough time to escape. Everyone present knows that Z has no chance of making it out alive, but Z fights on anyway. The scene is made even better and more powerful by the awesome music, Ocean's Guide, playing in the background.