When Black Bolt flew off to the moon to fight off against the Celestials while dying and screamed out for Galactus.
Iron Man in a super huge mecha suit fighting against the Celestials alone. It's a mecha that fires Iron Man suits as ammo.
The Celestials, walking the Earth, dwarf New York's skyscrapers as they prepare to scour Earth of all life. Then, a tiny silver figure on a surf board flies past them... and they gaze up to Galactus' face looming across the horizon. All of the horizon.
In Universe X, just as it looks like Death and Mephisto have put down Mar-Vell's rebellion in the land of the dead, The Cavalry arrives - in the form of Captain America leading a combined force of dead heroes and villains in a charge over a hill and towards Death's fortress, in a two-page panel of pure awesome.
If you're a fan of Magneto, there's something extremely awesome about the Oh Crap reaction out of Toad when he realizes they've swapped powers again. Eric is once again the Master of Magnetism.