Probably the final battle in Cowboy Bebop. No superpowers, no special techniques, just one badass taking out everyone that stands in his way.
hmmm I have to say Naruto vs Pain (but most of the fights are awesome) also Natsu vs. Jackal (again all of the fights are awesome)
I'm gonna go with the moment that any 'weak' lead character actually reveals their true strength. I just love how everyone's like, "You're weak, what are you gonna do?" Then, before they can bat an eye, they're knocked out cold. Aria the Scarlet Ammo is a good one for this type of scenario. Basically when the lead character looks weak but it turns out they were just hiding their true strength all along. Kirito from SAO would also be acceptable in early season one. -Nights