Alduin should be....for lack of a word....otherwordly. His body should pulsate with red/black lightning, and there should be a visible distortion around his body that warps everything around him...making it clear that he is like a force that does not belong in the world.
His flight should be much quicker and more graceful...and maybe a little unrealistic in how quickly or sharply his motions are, even leaving afterimages in his wake.
He should move not like a dragon, but like how someone "thinks" a dragon would move, like a god that has no idea how an actual dragon's body really functions, but is giving us their interpretation of what they think it is like.
He should move in ways that make you look and think..."that's impossible"...and regardless of his size, his physical force should be greater than even the largest dragon in-game if he isn't it. His dragon shouts, if he indeed even uses them should produce effects far beyond the maximum we or any other dragon can produce.
He should also have plenty of special attacks, abilities, and immunities that only he possesses. But most of all, his battle needs to be amazing to behold, and amazingly difficult. It needs to cement in our mind that, we barely survived, and that only the dragonborn could have beaten him.