I think RAGTIME definitely has one of the most successful and effective lengthy opening sequence. Not a minute of it is wasted and by the end of the number all the characters have been introduced, the time and place has been set, the mood of the show is established. Brilliant.
I don't find the song "Sunday In the Park with George" a lengthy opening but I guess that's subjective.
Another lengthy opening number that works for me is the "16 Feet Below Sea Level" sequence in CAROLINE, OR CHANGE.
A CHORUS LINE wins the award for me though.
"Some people can thrive and bloom living life in a living room, that's perfect for some people of one hundred and five. But I at least gotta try, when I think of all the sights that I gotta see, all the places I gotta play, all the things that I gotta be at"