In no particular order:
Usopp vs. Luffy - The emotion in this fight was so palpable and the creative way that Usopp was able to injure Luffy was amazing even though Usopp never stood a chance. Prime Oda here imo, I love this fight.
Franky vs. Senior Pink - Great back and forth between two manly men. The emotion of Senior Pinks backstory and the respect between these two makes this a jewel for me.
Sanji vs. Kuroobi - A great showing for Sanji imo. Blowing CO2 into Kuroobi's gills so he could get the fight back on land was ingenious and the combination attack was (and still is) amazing, also this is the first time Sanji showed off my favorite attack Mouton Shot!
Luffy and Law vs. Doflamingo - For having never fought together in the past their coordination against Doflamingo was amazing. Very high stakes and emotion.
Zoro vs. Mihawk - Probably the best squash fight ever while not taking anything away from Zoro's character. "Would you hunt a rabbit with a bazooka?" The epidemy of epic.
Honorable mentions:
Blackbeard vs. Ace, Usopp vs. Chew, Usoop and Chopper vs. Mr. 4 and Ms. Merry X-Mas, Usopp vs. Perona, Luffy vs. Bluno, Luffy vs. Arlong, Zoro vs. 100 Baroque Agents, Zoro vs. Mr.1, Sanji vs. Mr. 2, Luffy vs. Kaido (could move up).
Usopp fights are amazing!