GYPSY an CANDIDE are obvious choices but would still top my list.
Actually for me, just about ANY Jule Styne Overture (GENTLEMEN PREFER BLONDES, PETER PAN, BELLS ARE RINGING, SUBWAYS ARE FOR SLEEPING, DO RE I, FUNNY GIRL...) captures the brassy sound of Broadway at its best. There is an excellent collection of Jule Styne Overtures available:
http://castalbums.org/recordings/Everythings_Coming_up_Roses_The_Overtures_Of_Jule_Styne_Vol_1_Jack_Everly_and_the_RCA_Victor_Symphony_Orchestra/5086 Jerry Herman's MAME and MACK AND MABEL fit into the same category, and his DEAR WORLD is wonderful too.
The version of the Overture used at the start of the movie WEST SIDE STORY is terribly exciting. I also like the overture to WONDERFUL TOWN and the (apparently unused) overture to ON THE TOWN.
Technically "The Carousel Waltz" is not an Overture but on its own I rank it as one of the greatest concert music pieces of the 20th century. Rodgers' Overtures are all beautiful, especially SOUTH PACIFIC, and THE KING AND I. The Rodgers & Hart Overtures are not as well known but the ones for BABES IN ARMS, ON YOUR TOES and PAL JOEY are all great.
COMPANY doesn't have an Overture, but if you Arthur Fiedler's Fabulous Broadway to Hollywood CD you will hear a terrific 8 minute medley taken from the score of COMPANY that could very well serve as an Overture. The Overture from MERRILY WE ROLL ALONG is another favourite.
After seeing and hearing the recent revival I would have to include FINIAN'S RAINBOW: one great tune after another.
One other collection well wort checking out is taken from an Lp Lehman Engle conducted in the 1950s called Curtain Going Up. It preserves 10 classic overtures from BLOOMER GIRL, CALL ME MADAM, KISS ME KATE, ON THE TOWN, BELLS ARE RINGING, GENTLEMEN PREFER BLONDES, CAN-CAN, SILK STOCKINGS, WONDERFUL TOWN, and FINIAN'S RAINBOW. (The CD adds two bonus tracks of LI'L ABNER and GOLDILOCKS.) It is out of print now but turns up frequently in used CD stores and on E-Bay.
http://castalbums.org/recordings/Curtain_Going_Up_Overtures_of_Broadway_Hit_Shows_1958_Broadway_Overtures/6797
Cast albums are NOT "soundtracks."
Live theatre does not use a "soundtrack." If it did, it wouldn't be live theatre!
I host a weekly one-hour radio program featuring cast album selections as well as songs by cabaret, jazz and theatre artists. The program, FRONT ROW CENTRE is heard Sundays 9 to 10 am and also Saturdays from 8 to 9 am (eastern times) on www.proudfm.com