Boys
That being said, the fights that come to mind for me:
Yu Yu Hakusho (duh they're Kurama fights )
Kurama vs. Touya - The battle of two smart, sneaky fighters. Kurama's already the underdog with his energy sealed up from his previous match and things look bad for Team Urameshi, but Kurama uses his smarts and self-sacrifice to beat Touya, who I'm also rather attached to.
Kurama vs. Karasu - I've always found the Karasu match to be rather interesting because I'm cruel and like seeing my favorite characters squirm *looks around* I didn't mean it like that! Anyway! I seem to recall this fight won favorite fight in the series at some point and with the way Karasu gets under the ever-calm Kurama's skin, I can see why. Giant, man-eating plants, borabs, blood, gore, death, and Youko Kurama... all kinRAB of good stuff going on.
One Piece
Straw Hats vs. Kuma - When a fight makes me cry both in manga and anime form, there's something amazing going on. This is less of a fight and more of a complete and utter rout, but that makes it stand out even more for me... and we're still seeing the ramifications and will be for awhile. Just wow.
Naruto
Naruto vs. Sasuke (Valley of the End) - Leave me alone, I like Sasuke and Uchiha backstory.