This is a favorite subject of mine...
I agree with some (but not all) of the choices that have been mentioned, but I would specifically cite:
PHIL LANG - too many great sets of orchestrations to mention, but some of my favorites of his are the charts for High Button Shoes, Goldilocks, Jamaica, Li'l Abner, Take Me Along, Carnival, Subways are for Sleeping, Jennie, Mame, I Do! I Do!, How Now Dow Jones, Sugar, Mack and Mabel, Annie, 42nd Street. His work has that traditional Broadway sound, but his writing is always layered and interesting, with incredibly colorful little details.
EDDIE SAUTER - all of his shows have a unique, idiosyncratic sound. It's a Bird! It's a Plane! It's Superman, Henry, Sweet Henry, The Apple Tree, 1776 and Georgy sound nothing like each other, but are tailored specifically for the score's individual needs.
RALPH BURNS - wrote the swinging charts for No Strings, Funny Girl, Little Me, No No Nanette, Chicago, Sweet Charity, etc., but could also do the traditional Robert Russell Bennett sound better than Robert Russell Bennett in such scores as Do I Hear a Waltz? and Darling of the Day.
ROBERT GINZLER/SID RAMIN - together and apart, are responsible for West Side Story, Gypsy, Wonderful Town, Bye Bye Birdie, How to Succeed in Business, Wildcat, Forum, Irma La Douce. Ginzler's orchestrations for Birdie and How to Succeed are some of the finest orchestrations ever written.
DON WALKER - lots of great work, but some of my favorites of his are Carousel, The Most Happy Fella, Anyone Can Whistle and The Rothschilds.
JONATHAN TUNICK - I think he's gotten less interesting in recent years, but Promises, Promises, Company, Follies and A Little Night Music are sublime.
ROBERT RUSSELL BENNETT - had a reputation for writing somewhat square orchestrations, but I think his work for On a Clear Day You Can See Forever is perfection.
HAROLD WHEELER - one of the best orchestrators still working, and responsible for the terrific orchestrations for Dreamgirls, The Tap Dance Kid, The Full Monty, Hairspray and Dirty Rotten Scoundrels. He's able to write in a pop idiom, but still achive a theatrical sound, not an easy balance.
LUTHER HENDERSON - terrific orchestrator and dance arranger. Love his work on Do Re Mi, and those incredible dance arrangements for No No Nanette.
Begin at the beginning and go on till you come to the end: then stop.