The opening cutscenes give you a good deal to go off of. Okay, sure, this guy is a Badass Longcoat, I get it. Nigh invulnerable, uses two guns, nice... It's all very nice, seems like it is ramping up to be a cool action game. Then you see this scene. There's a point during it when you think to yourself, "No, he's not going to do that!" But he does. This was the make-or-break moment: if you weren't mesmerized by the sheer coolness of that scene, chances were this wasn't the game for you.
Right before the first fight against Mundus: 1:25 in, full stop. After about twenty levels worth of badass taunts and one liners, we get to see what Dante's like when he's really pissed off. No joking, just a Berserk Button being pushed and a badass, three-part fight following.
For the player: surviving the entirety of Dante Must Die mode in the first game would cause Dante to have some of his dialogue change during the ending sequence, along with a remixed version of the result screen theme. You're then rewarded with Super Dante - picking him on a New Game means he has his base stats and weapons, but when you get Alastor and Ifrit, this Dante has infinite Devil Trigger!
Trish driving a motor cycle through Dante's door. She then shows her skill and power by knocking him down through a combination of martial arts and lightning. Granted Dante does recover rather quickly and turns the fight in his favor but it's rather impressive for Trish given who she was dealing with. Her Taking the Bullet for Dante and later on lending him her to defeat Mundus also qualify.