I'd have to say that big 8-page splash in volume 2.
With Quicksilver appearing at least once on every page.
I'd say even right before it, with Thor:
Thor: "Did you honestly think I would come here alone?"
Cap impaling Red Skull with an airplane.
Did we say airplane? We meant teleporting fighter jet.
"Yeah, well Darth Maul wants his *** lightsaber back!"
After being betrayed by his fiance Black Widow Tony Stark stalls her for twenty minutes, long enough for the nanites in her brain and blood (put there so she can control her own suit of armour) to download everything she knows about the plot against the Ultimates to Tony's brain. Then he uses the nanites to give her brain damage, smashes her head in with a champagne bottle and gets into action.
Captain America's Super Serum blood kicking the crap out the vampire infection when a horde of bloodsuckers descend on the Triskelion at night. What is Cap's plan of attack? Grab Thor's Hammer and teleport the entire Triskelion to the middle of sunburnt Iran, instantly torching the entire vampire army.
Cap absolutely demolishing Hank Pym, who was on the run after nearly killing Jan. The best part is Cap forced him to grow so he wouldn't be beating on a helpless man. And he still did it without breaking a sweat, just by using the environment to his advantage.
At the climax of the first volume Hulk curb stomps The Colonel then starts trashing alien ships without Thor's lightning or Iron Man's weapons.