A really well written or well staged opening number can jolt a show to life... A few that come to mind...
"Cotton Blossom" from SHOW BOAT
"He Had Twins" from THE BOYS FROM SYRACUSE
"This Time of the Year" from FINIAN'S RAINBOW
"Runyonland/Fugue for Tinhorns" from GUYS AND DOLLS
"A Typical Day" from LI'L ABNER
"Little Old New York" from TENDERLOIN
"Comedy Tonight" from A FUNNY THING ... FORUM
"How to" from HOW TO SUCCEED IN BUSINESS WITHOUT REALLY TRYING
"Wilkommen" from CABARET
"Company" from COMPANY
"I Hope I Get It" from A CHORUS LINE
"All That Jazz" from CHICAGO
"20 Million People" from MY FAVORITE YEAR
"Circle of Life" from THE LION KING
"Good Morning Baltimore" from HAIRSPRAY
"Gif me the cobra jool!"