Kitty Pryde going giant size, thanks to Pym Particles, and fighting Galactus.
The amount of effort the heroes put in stopping Galactus is awesome in itself.
Despite never having to have to fight something like Ultimate Galactus before. A fusion of two realities Worldbreakers and cosmic horror. The Ulitmate universe managed to cross realities, forged new alliances and technologies, and flung themselves fearlessly into the fray to save their broken World.
This is also a moment for Galactus himself, who after years of getting used for The Worf Effect makes a roaring comeback. Not only is he the Big Bad once more (or at least the biggest threath), he actually Took a Level in Badass by fusing with his Ultimate counterpart!