Oh so many moments to count! But I do have one awesome moment that comes to mind. *spoiler alert* After freeing Odahviing from Dragonsreach in Whiterun, you learn a shout that summons him to fight with you. I remember testing it out once or twice, nothing special. I killed mostly everyone before he got there. :/
So one day I was roaming around in The Rift, discovering locations so I could explore them later. As I was walking across this open field I hear an Ancient Dragon screech above me and then its immense shadow pass over. Usually when you shout at a dragon it lands to fight, or circles back around to attack you, so I shout "OH DAH VIING!"; it was selected already for some reason. That little 'Aha!' moment popped in my head so I ran for cover as this battle was going to be epic. In the distance I see him (Ohdahviing) flying towards me. he sees the hostile dragon trying to kill me and swoops by him, slashing at him with his claws. (I'd never seen that before!) They fly out and circle around, coming in for go at each other. This is where it happens. The two dragons fly head on towards each other, a little offset like jousters from medieval times. Then "YOL TOOR SUL!" (fire breath shout) amplified, and slightly echoed, by both of them shouting at the same time. As they shouted, the pillar of fire that came from each dragons' mouth, seemed to connect them and turn as they circled in midair, right above me. I let them fight and Ohdahviing proved victorious, killing the other dragon in midair, where he crashed to the ground and died. Then Ohdahviing flew off letting me claim another soul!
Just epic, like right out of a movie. I've tried this multiple times, but its never an epic of a battle as the first time.