Vash ensuring there were no fatalities in a quick draw shooting contest by standing on the sidelines and flicking rocks into the bullets to change their trajectory to a nonfatal one.
Nicholas D. Wolfwood shooting a machine gun out of one competitor's hands with one bullet... and lighting said competitor's cigar with the next!
In Vash's final fight with Knives, Knives throws five bullets in the air and then shoots them causing them to explode in unison all over Vash.
A defining moment for Vash, where he is revealed to be something more than just a lucky moron, occurs in the second episode where he stands perfectly still as a gun is leveled at him and fired and then dodges several bullets.
The entire first five episodes are a collective crowning moment. Think about it. Vash never fires a shot for the first four episodes, and easily defuses several extremely dangerous situations. And when he does finally fire his gun, it's a single, quick moment that completely shuts down the enemy's ability to fight back. Bad. Ass.
"Total slaughter... total slaughter... I won't leave a single man alive..."
Vash has a missile headed straight towards him. His solution: kick that fucker out of the way.
Vash stripping naked and barking like a dog to save someone's life. Hell—sure, he can blow a hole in a moon with his Angel Arm but only a true badass can unashamedly beg to save someone's life.
Then, upon finding the hideout of the men who have been harrassing his adoptive family for the whole chapter, he and Wolfwood proceed to take them all out. It's even better in the manga, though, when Vash and Wolfwood find the Legendary Impostor who's kidnapped Vash's adoptive sister. He tells Vash to drop his gun after some bragging, which Vash does... only for him to pick it up, in midair, and then riddle him with bullets before he can even react.
Legato got a crowning moment of awesome when finally, after all the efforts of the Gung-Ho Guns to cause Vash pain had failed, he succeeded by methodically forcing Vash to shoot him execution-style in the head.
In this contributor's opinion, Wolfwood's most awesome moment is after he's dead. In the last episode, after Vash and Knives joust with their angel arms, leveling the terrain, Knives manages to get Vash's revolver and activate both angel arms while Vash is helpless and exhausted. With Wolfwood's voice urging him on, he lets his hand fall into the dust beside him... and lifts up Wolfwood's cross machinegun, buried in the dirt, which Knives (and probably the viewer) forgot was there. He shoots at one of the angel arms, causing Knives to lose his grip on both weapons and allowing Vash to make a comeback.
Wolfwood ups the level of awesomeness to gratuitous amounts in volumes 9 and 10 of Trigun Maximum. In order to save the orphans he set out to protect in the first place, he firsts defeats a small army of cyborg mercenaries, then Livio the Doublefang, who was basically stated to be an upgraded version of Wolfwood. He then overpowers the True Chapel only to get attacked by Livio's split personality, Razlo the Trip of Death, who uses three of the giant cross guns. He takes a bit of a beating here, but Vash comes in and stalls them long enough for Wolfwood to get back to his feet. Chapel, not being an idiot, starts shooting Wolfwood, so Wolfwood BITES though two glass vials of super soldier formula, heals as he's being shot, and promptly head-butts Chapel so hard that his neck snaps. As if that wasn't enough, he then defeats Razlo the Trip of death by exploding a freaking chain of bullets around his head after disarming him at close range. Sure he died, but it was just about the best way anyone could possibly good out. Crowning Moment of Awesome indeed!
One instance in the anime is when Millie fires her stun gun at Monev. He shrugs it off, but still, she's got ladyballs for attempting it.
Vash's fight with Gasback at the end of Badlands Rumble.