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 Herr Kleiser then starts trashing alien ships without Thor's lightning or Iron Man's weapons.
Cap's response upon being asked by Herr Kleiser to surrender.
Captain America: Do you think this letter on my head stands for France!?
Although relativized by Cap at the end of the first Ultimates volume (He called this remark stupid and attributed it to the heat of the battle). Nevertheless it is stated in the second volume of the Ultimates that he does hate France.
Which was later parodied by Elsa Bloodstone in Nextwave, who was wearing a European Union t-shirt.
Elsa: Victim? Do you think this letter on my chest stands for America!?
Wasp crawling through the Hulk's ear in order to administer a much needed wasp sting directly into his brain.
Although later events somewhat change one's view of the character, the first appearance of Ultimate Black Widow included a scene where she jumps across a street in New York between skyscrapers, grabs a sniper rifle in mid-air that was dropped from a helicopter, and slides to a halt in the building on the other side while taking down the bad guys and saving Hawkeye.
Bruce Banner being thrown out of a helicopter to make him turn into the Hulk
—> Thor: "Did you honestly think I would come here alone?"
"Yeah, well Darth Maul wants his *** lightsaber back!"
After being betrayed by his fiancé 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.
A small moment of awesome for Natasha, since she manages to figure out how to remove the nanites and escape while everyone else is busy. Then an angry Hawkeye finds her.
The "Grand Theft America" arc has many, especially during the triumphant climax.
Captain America bursting free from the grasp of a dozen super-strong foes.
Wasp using her "sting" at full size to cave in a bunch of dudes' faces.
Iron Man proving that he's still a very good shot, even while drunk.
Quicksilver - Getting up after being blasted by lightning four times to launch himself hundreds of feet into the air and save the day by snatching Thor's belt. Evacuating the top three floors of SHIELD headquarters in the blink of an eye. Gruesomely dispatching an enemy speedster in between her last attack on Hawkeye, and Hawkeye hitting the ground (Hawkeye, of course, sees Quicksilver slumped over sweating and asks "Hey, quit messing around and give me a hand here")
Ultimate Hawkeye had already proven himself a true badass, but working his own fingernails off and flicking them into the faces of his captors causing them to choke while tied down to a chair, and thus allowing him to make an escape, took him to new levels of pure awesome badassitude.
Scarlet Witch has two, in separate instances against Loki and Thor:
Against [ Thor]: "Somewhere there's a reality where you are a baby seal being clubbed to death. I'm going to find it and bring it here."
"I'm increasing the odds of someone showing up to kick your [ Loki's] ass".
Ultimate Thor, while facing down Loki's army of demons without any glimmer of hope, calls down the Bifrost rainbow bridge, the gateway to Asgard, down which storms an entire army of Viking warriors straight from Valhalla. They then proceed to beat the crap out of every baddie in town.
Bruce Banner revealing that he survived his attempted execution (electric chair? lethal injection? no, a nuke). How did he reveal this? By being stepped on by a robot, transforming into the Hulk, then proceeding to toss the robot - hundreds of times his size, even as the Hulk - around like it was nothing.
Hulk tearing apart his Evil Counterpart ("YOU THINK TOO MUCH!"). He literally punches Abomination's head off!
One moment for Fury occurs in an annual set during the timeline of this story. An assassin is called out of retirement to kill Nick Fury. He is given a special gun, of which only two were made, that can both see through and shoot through walls, the assassin tracks Fury, sets up in a nearby building, and as he sets the scope to point at Nick's exact location, the last thing he sees is Nick firing the other special gun at him. It turns out, Fury put the hit out on himself to draw this assassin out of hiding.
The European heroes, led by Captain Britain, who sneak into the country as civilians and cheerfully explain the concept of "secret identity" before opening a can of whoop-ass.
Hawkeye taking down a squad of highly trained soldiers by throwing dinner knives and forks. Then after he's captured and tied down, escaping by tearing out his fingernails and flicking them into enemy throats.
Finally, an eight page splash panel of everyone fighting an army of orcs and gigantic wolves, with Quicksilver on every page of the splash, just to reiterate once again how awesome he is.
The splash panel with just about everyone in the Ultimate Marvel universe kicking ass and taking names from the Liberators.
Cap impaling Red Skull with a teleporting fighter jet.
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.
Surprise! The Iron Man Six armor! This is after he found out that Natasha was a spy - he found this out by her killing Jarvis - and that terrorists had invaded. He responded by whacking her with a bottle, getting very, very drunk, and launching, in his regular armor, for the space station. His next appearance is in the Iron Man Six armor, laying waste to the Crimson Dynamo robots.
Hawkeye's escape scene in Grand Theft America where he kills a room full of soldiers by ripping his fingernails out and flicking them at them, strapped to a table whilst doing this. Followed up by his turning to the security camera which had just seen him doing this, and grinning a Slasher Smile: "Run."