The Metal Scream from "The Number of the Beast." It's so good that Bruce has never been able to replicate it since doing it for the first time, he can only come close to replicating it.
Made even better by the fact that Bruce has gone on record saying the scream was a result of his frustration with the song's producer, who kept making him redo the song's intro, which had the tricky time signatures.
Somewhere Back in Time tour. 45 days, 23 shows. See Flight 666 for details (and some excellent footage from 16 of those shows.)
Also awesome: Bruce Dickinson flew the tour plane on that tour and is a commercial airliner pilot. Yes, he has two of the most stressful jobs in the world.
Iron Maiden in 2008 playing in India, 30,000 people packed the stadium for the first big metal band to ever play in the 2nd most populated country in the world. History was made when the devil horns were thrown up in the whole crowd rocked out.
Live After Death. Full Stop.
Any time Eddie makes an appearance during one of their shows.
Let's go to the point: anytime Iron Maiden appears on a stage is a CMOA. The band is thrice-distilled, pure awesome. Want proof? Watch Flight 666. There is a priest out there, who bases his sermons on their lyrics.
In 2006, at the Nassau Coliseum (Uniondale, NY) gig of the A Matter of Life and Death World tour, some dumbass thought it would be a good idea to bring a sign that read "Play Classics!" for a tour where Maiden would play the eponymous album. During "For the Greater Good of God", Bruce finally noticed the dickhead with the sign and tore it apart. While on the bootleg all you can see is Bruce having the dick hand him the sign, this video allows you to see the badassery from the front row.
On that note, any time Bruce yells at a dickhead in the audience.
Maiden, who are older than Metallica, are open about how they don't mind people downloading their music for free at all. In summer 2003, Maiden toured America and select countries in Europe, a few months before Dance of Death was released, playing one song from the album. Bruce tells the crowd that as long as they buy the album, they don't give a fuck about people downloading their music.
Nicko once played for Pat Travers in the seventies, and admits to hating it to the point where "I wouldn't stop to piss on him!". At the 2009 premiere of Flight 666 in Rio, a dumbass yank in the audience asked Nicko a question about Pat Travers. Nicko lashed out against him and had him removed from the cinema.
Similarly enough, he ran a security guard over in a Dallas parking lot in 2003 prior to a show in Dallas. Which inspired a bootleg called "Running you over!"
At their concert in Milan in 2013, during Aces High, Nicko hit the snare drum way too hard thus breaking it. Instead of having the band stop the song, he played the snare parts on his lower tom until a roadie took the broken snare and put a new snare in its place. Watch the awesomeness here.
Bruce would then introduce The Evil That Men Do by saying, "The evil that men do lives on... But the good is often turned with their bones... and so is the snare drum.
"The Thin Line Betwee Love and Hate" from the Brave New World album. Maiden's first album with their modern line-up, with Bruce and Adrian returning after 7 and 10 years respectively. At the very end of this 8 minute+ song, Nicko can be heard saying "Oh, I fuckin' missed it."
The "Final Frontier" music video pays homage to Alien among other space operas and horror films, and has fantastic cinematography and special effects to boot.