Whenever they make multiple acts perform together, pure awesome is being made.
In Season 2 with acts that didn't make it far performing "We are the Champions" and in Season 5 when Studio One Young Beast Society, Michael Grasso, Anna and Patryk, Michael Lipari and Ashleigh Dejon, Haspop, and Harmonica Pierre performed all at once.
In Season 5, a Elvis impersonator in clownface (really) was buzzed, and he made a joke about Sharon getting a "contact high" from living with Ozzie. Sharon's response: "Nobody disrespects my husband. Fuck off." They then moved on insult Howie Mandel's career...causing Howie to repeat what Sharon had said.
Recent evidence suggests the guy is actually the lead singer of a band who came on the show specifically to troll the judges, though. Doesn't make it any less awesome.
Season 6, the Kinetic King's Epic Fail where his trick didn't work at all, and he was eliminated from the competition. Then the week came where twelve eliminated acts came back for another chance, and he came back. He noted before it aired that he was so concerned with what happened last time that he spent a lot of time making sure nothing could go wrong—in the accompanying montage he was shown looking up weather patterns. The research paid off though, because his trick went brilliantly and he received mass applause and a spot in the Semi-Finals.
Season 7, William Close and his freaking huge "Earth Harp". Not only does he make instruments, he plays them so damn well that it's incredible to watch. His performance during the quarterfinals playing "Love, Reign o'er Me" by The Who with his harp, a group of people performing with him and spinning drums was just nothing short of amazing.
To clarify, the aforementioned "Earth Harp"? It's essentially a giant violin that uses over 3000 feet of string, and uses the theater it's used in as the setting. In other words, he turns the theater itself into an instrument and then plays it.GENIUS.
Tom Cotter managing to finish in second place overall in season 7. The awesome part about that? He is the first pure stand-up comic to even make it to the finals, with the only earlier one (Terry Fator, season 2's winner), arguably being more of a singer than a comedian.
Taylor Williamson, who was actually on a previous NBC competition series before, also surprisingly finished as the runner-up for his season. His hilarious post-routine banter with the judges, especially involving Heidi Klum, certainly helped him win over fans.
In Season 8, Brad Byers, who lost a previous season, returned with an act that in an of itself deserves an entry here. He swallows nine swords, then twists them. He even uses an X-ray machine to prove it's real. It was so amazing, a judge that admitted she would've no, said yes.
Also in Season 8, Kenichi Ebina and his wickedly creative, unbelievable popping-style dance moves. His quarterfinals performance was especially top-notch, where he plays the role of every character in a fictional video game. The real him (in front of a projected screen) fights a bunch of characters (on said screen) that whittle down his health until it becomes Game Over for our dancing protagonist. Then, the "game" rewinds including Kenichi himself. When the rewind stops, he grabs a sword, and defeats the final boss to win. Even with this description, words cannot describe how amazing this is, other than that Howie is 100% right on saying that Kenichi has to win this season. And he does!
Howie Mandel: ďIíve never done this before in four years on this show, but Iím begging you, Kenichi has to win the million dollarsÖ Itís the best act Iíve ever seen.Ē
Kenichi tops himself again at his first performance at the finals, a touching story of himself dying and ascending to heaven, where he, through the use of stroboscopic lights to freeze his moves, managed to give the impression of flying through the stage.