Mama Rose in GYPSY, as many others have already mentioned. Even Ethel Merman, who was known as being calm, cool and collected on stage, used to sit in her dressing room during intermission, utterly exhausted with her head in her hands, awaiting the 2nd act with her demanding 11:00 number(Rose's Turn) still to come.
I also agree with Bobby in CRAZY FOR YOU. He is onstage almost the entire time. Harry Groener was just wonderful in that role--an under-rated actor/singer/dancer IMO. Where is he now?
"Madam Rose...and her daughter...Gypsy!"