If you take your time handling Hewdraw, Palutena will actually pitch in and help you, since the Underworld's Forces are invading Skyworld. Normally Pit would have to fill the monster up to their throats with bullets. Palutena simply drops a HUGEPillar of Light and oneshots them.
Dark Pit's introduction during the battle with Pandora, especially when he knocks her down with a single punch!
The Space Pirates. They go around stealing constellations and then sell them on the black market or use them for fuel.
They also put up an impressive fight against the Underworld forces. One scene in particular shows a pair of Space Pirate Snipers fighting a Clubberskull! The Snipers win.
Dark Pit knocking out the Underworld Gatekeeper (a giant flying robot) with a side kick from above.
Throughout the air section, you are treated to an awesome backdrop of Thanatos and Phosphora duking it out.
The air section in general. The chapter starts off with the door getting blown up by a laser. It only gets better from here, as tornados rise from the clouds and this absolutely EPIC theme starts blasting as you pummel both Underworld Troops and the Forces of Nature.
Hades of all people gets one in-universe when he throws an Aurum ship into their own mothership.
The beginning of the chapter, where Pit, the Underworld Army, the Forces of Nature, and Pyrrhon all teamed up to take down the Aurum brain. Pit notes that everyone is in good spirits.
When Pit's Power of Flight runs out, Viridi comes to your rescue by having some of her troops carry a chunk of rock to serve as a platform, culminating in the troops performing a Heroic Sacrifice to save Pit from Pyrrhon's massive column of fire.
The Centurions of all people save you from falling to your death again by carrying another chunk of rock to serve as your platform while helping you fight off some of the Aurum. The music certainly helps.
Hell, Pyrron hijacking the Aurum brain and turning it against all the other Gods was pretty friggin' awesome by itself. Especially when you consider what a(n apparent) blowhard he is.
The entire level is a Mêlée à Trois between the Underworld forces, an army of Centurions led by Pit's brainwashed body, and Magnus. The fact that he even registers as a threat alone would be a Moment Of Awesome for the character, but winning makes it even better.
Pit's response to facing someone so powerful that they lazily toss islands at him? "Send me in there".
Pit: Go home, Hades. Just go home.
The latter half of the aerial section where Pit is chasing the real Chaos Kin. The combination of all the stuff flying at you, the sense of urgency, and the Awesome Music make this part incredibly epic.
The end where Pit chooses to save Dark Pit at the cost of burning his own wings. Dark Pit's response is shown in the next chapter where he decides to dive into the Underworld to save Pit.
Dark Pit's entrance during the land battle sequence? You're making your way through wave after wave of enemies that seem to never end. You're probably getting tired of having to deal with all of them, and it gets to the point where the game has to cut to a loading screen to keep summoning enemies... And then you get a quick scene of Dark Pit arriving on scene. It suddenly cuts back to gameplay with a snippet of the Chapter 6 Air Battle theme playing as Dark Pit begins slaughtering enemies with you. Not to mention Pit and Dark Pit's pre-batle rallying cry.So awesome.
In Thanatos Rising part 3, despite Thanatos having the advantage over Pit and having a huge opportunity to kill him, he decides to toy with Pit and throw spike bombs while Pit hanging over the edge. Not only does Pit dodge everything thrown at him while still holding on to the ledge but he uses his other hand to shoot the gear above him effectively defeating Thanatos when it falls on him. And this is while Thanatos is taunting him.