Day 21: Luffy vs Lucci @ Enies Lobby without a doubt.
It still stands to be one of Luffy's most impressive displays of his determination, skill, strength, and willpower. His role as Captain was justified 1000 times over because he was able to endure his most exhausting battle he's ever had so far, protect his crew, and rescue Robin. The stakes were higher than they've ever been at that point in the story and for a long time it really seemed as though Luffy had finally met his match.
Plus, Mayumi Tanaka's final yell during the Jet Gatling never fails to give me chills and expertly showcases the desperation and passion Luffy had put into that last attack. Luffy was so far beyond his limit and he absolutely NEEDED the Jet Gatling to be enough for the finishing blow otherwise he'd lose everything. Overall it just really blew me away when I first saw it and still does to this day and made me really admire Luffy's character.
Since he first popped out of that barrel during his humble days as a common pirate, who would've thought he'd become such a notorious captain and end up challenging the entire World Government while simultaneously defeating one of it's most powerful and legendary agents by himself?
The whole fight really just put Luffy's entire adventure so far into perspective for me.
TL;DR: That Lucci fight was pretty cool man…
HM: Luffy's entire struggle to save Ace, since Amazon Lily all the way to Marineford. It was Luffy vs. the World basically.
Luffy fight in a Movie: Luffy vs. Zetto