Ragtime is my absolute favorite. The way it establishes all of the characters and racial/ethnic groups is so well-done, and the choreography and characterization go so well with the music.
Who would I be if I didn't include Anastasia somewhere on this list? The Dance of the Romanov's is absolutely gorgeous.
I thought this would be a hard choice - all of the numbers mentioned start their shows with a bang. But then I remembered "Circle of Life" from The Lion King and realized that there is actually one opening that is head and shoulders (and trunk and hoof) above anything else - in my opinion, of course.
I may be bias because it was the first show I saw on Bway, and I started crying as soon as the curtain rose, but Welcome to the Renaissance from Something Rotten will always hold a place in my heart.
I have a soft spot for openings that introduce characters/the plot in a summary or list a la "Tradition" from Fiddler on the Roof (so "Prologue" from Great Comet and Ragtime, "In the Heights" from ITH and "Alexander Hamilton" from Hamilton I suppose all fit the bill).
"Just Another Day" from Next to Normal.
"Prelude/The Ballad of Sweeney Todd" is always a killer.
Circle of Life from The Lion King is my favorite. An absolutely stunning 6 minutes of theater. Shame the rest of the show can't live up to it.
I also love the opening numbers from Ragtime and Titanic because of the way they seamlessly integrate exposition with sensational singing on a large scale. I'm generally not a fan of opening numbers where characters come out and introduce themselves to the audience, but it works perfectly here.
I also want to shout out musicals that do not start with an opening number.
I think the best of those is DEH. Evan's nervous jittery monologue is absolutely perfect for setting up his character, and it leads beautifully into his I Want song "Waving Through a Window."
Most of my favorites (because "Best" is so subjective) have been mentioned.
I am partial to "I Hope I Get It" (A Chorus Line) and "It's Your Wedding Day" (Wedding Singer) because how they just explode off the stage. "Fugue for TinHorns" (Guys and Dolls), "Tradition" (Fiddler) (and it's offspring "In The Heights" and "Comedy Tonight" (Forum), to me, are perfect introductions to shows. "The Ballad of Sweeney Todd" is eliminated because of that *@#$%^& factory whistle. And, the off stage beginning of "Oh What A Beautiful Mornin'" (Oklahoma), even today, seems fresh and different.
I guess i can toss in "There You Are" (The Mystery of Edwin Drood).
Prologue - Ragtime Circle of Life - The Lion King Tradition - Fiddler on the Roof The Carousel Waltz - Carousel It's Your Wedding Day - The Wedding Singer Magic to Do - Pippin Alexander Hamilton - Hamilton Audition - 42nd Street Prologue - Into the Woods Runyonland - Guys & Dolls