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).
So, I guess top Five:
5. Oh What A Beautiful Mornin'
4. Comedy Tonight
3. It's Your Wedding Day
2. Tradition
1. I Hope I Get It