To my mind, The Eve of The War is probably the greatest opening to a musical known to man. It obviously helps that it's based on one of the most influential sci-fi books yet written, as well.
Jeff Wayne's masterful opening composition from The War of The Worlds featuring Richard Burton and Justin Hayward as the journalist.
From Jeff Wayne's Musical Version of The War of The Worlds (2006): The live stage adaptation of Jeff Wayne's 1978 bestselling album, brought to life at London's Wembley Arena. Featuring performers from the original recording, including Justin Hayward, Chris Thompson Wayne himself and the sight and sound of Richard Burton, as well as performances from Russell Watson and Tara Blaise.