90% of the battle against The Dragon. Sure, it starts off a bit weak with you running away as he chases you, but he will eventually get stuck, allowing the Arisen to beat on him until he breaks free. He then fights the Arisen on the ground for a bit before taking flight and tearing the castle apart, leaving the Arisen with no option but to run along collapsing walls, making for a ballista to get the Dragon back into striking distance. After taking a ballista bolt to the chest, the Dragon is unharmed, but really angry, so he dives at the Arisen, who promptly jumps onto the Dragon to transition the fight into phase three, which consists of a Colossus Climb towards his heart, while he tries to shake the Arisen off by flying through canyons, flocks of harpies, and even above cloud level. Upon striking the heart, the Arisen rides the Dragon's stunned form back down to the ground to finish up the battle in a massive crater as reality itself starts tearing at the seams.
The Dragon is something of a One-Scene Wonder, in that while he only appears a handful of times throughout the game, every encounter with him is memorable, and more than that, everything he says is pure undiluted awesome. His earth-shaking Badass Baritone is truly what clinches it. A true hats-off to the voice actor.
Fighting the Ur-Dragon would surely count as well. While it doesn't play itself like a series of action sequences culminating in a fight, like the Dragon, it does make Colossus Climbing a necessity. It has weak points on essentially every part of it's body, all of which need to be destroyed to kill it. The fact that it's huge and, when you damage a weak point, it's skin gradually gets torn away, making it look like a zombie dragon, just adds to the spectacle. The desolate battlefield with patches of purple flames is also a brilliant stadium.
Warning: This is a major spoiler, but also a moment of incredible awesome. You are warned. Facing down Savan the first time makes for an incredible fight. But go through the game on New Game+, and the Seneschal this time... is YOURSELF. Your Arisen from the first playthrough stands before you. You knew it was coming, and yet it remains truly awesome.
Can be a Negated Moment of Awesome if playing in online mode: the Seneschal can be any other player's Arisen, no matter how terribly out-of-place they look. Imagine watching the hood come down off of what should be your old character's face and seeing... anime two-tone eyes and bright pink pigtails. 'Nuff said.
Two words. Blue. Afro.
The end of the Deny Salvation quest,specifically the moment when the Dragon crushes the Elysion in the middle of his speech. Not only is it satisfying to see the Smug Snake get reduced to paste in an instant, but the Dragon's next words sum up his importance in the grand scheme of things.
Dragon: The rantings of an upjumped zealot make for tedious listening. His ilk serves no role in what is to come.
When fighting the Archydra, it's very likely that all of your pawns will be swallowed, leaving you all by yourself, possibly for the very first time since the game started. Yet, if you use careful tactics, timing, and have enough patience, powerful enough spells or weapons, you can take down the beast by yourself! And considering how you've had pawns helping you out in every single battle before (unless they were killed), it's extra-awesome knowing you can take down such a leviathan without any help.