I think most of the big moments, that I like, have been mentioned... but...
I like it when Luffy does something completely unexpected because he doesn't agree with a plan or situation.
For instance, when he went to sleep in the Arlong arc or when he quit in Alabasta because Vivi wasn't williing to risk their lives. Luffy instinctively knows the best course of action, and that's what makes him a great captain.
Oh, and 4 words: Gomu Gomu no Baka!