Since you've defined the question so broadly and to include anti-heros and seriously flawed characters (who could argue that Billy Flynn isn't a seriously flawed human being?), many, just looking at the guys:
(note, of course, that many of these are of the musical comedy lovable bad guy, heavy, or pain in the butt type; others are nobly anti-heroic and driven; one or two are down and out nasty guys)
Ray Walston in Damn Yankees
Jonathan Pryce in Miss Saigon
Norbert Leo Butz Dirty Rotten Scoundrels
John Lithgow The Sweet Smell of Success
Bert Lahr in Foxy
Ben Harney in Dreamgirls
James Naughton in Chicago
Brent Carver Kiss of the Spider Woman
Len Cariou in Sweeney Todd
Barry Bostwick in The Robber Bridegroom (at least the Robber part of the title role)
featured:
Myron McCormick in South Pacific
Yul Brynner in The King and I
Cyril Richard in Peter Pan
David Burns in The Music Man
Jack Cassidy in She Loves Me
Hiram Sherman in How Now Down Jones
Rene Auberjonois in Coco
Keene Curtis in The Rothschids (some of his roles)
Henderson Forsythe in The Best Little Whorehouse in Texas
Chuck Cooper in The Life
Shuler Hensley in Oklahoma
Chuck Cooper in The Life
Updated On: 4/5/12 at 09:52 AM