Prisoner of Azkaban: Harry, Ron and Hermione vs Snape. 3 books of Snape being a git, then he gets knocked out. Comeuppance.

 

Umbridge does have magic, but let's just assume that Bane would somehow win. :awesome

Or maybe he just 'breaks' her wand. Kind of funny to imagine snapping a wand with all the drama of Bruce Wayne's back.

 

The fight against the Basilisk is all kinds of epic and literally the stuff of fairytales and legends. Canonical 12-year old Harry Potter was a badass. After that, the Graveyard duel was pretty gripping. While not a duel per se, Harry walking to his death in DH was a really powerful moment, even if the Elder Wand hax was a let down later. (I think Rowling could have justified Voldemort's spells also not working as well against Harry to give him an edge and it could have come down to yew vs holly again)

 

I am going to second the fight in the Ministry. Dumbledore vs Voldemort, the two wizarding titans of their world going all out, showing what a wizard truly is capable of.

 

Personally, the battle in GoF was my favourite as even though Harry was at such a disadvantage by already being exposed to Voldemort's crucios. However the magic in the duel between Dumbledore and Voldemort was the most amazing in the canon bar none.

 

People have said this, but I too like the graveyard fight in the canon....and in the Lie I've Lived too, or rather the lack of fight there.

 

I'm going to go outside of canon and say my personal favorite to read in fanfiction is Dumbledore vs Grindelwald.