Here's my list of musicals that are, IMHO, just plain bad. Awful. Horrible. Nearly irredeemable. Let's just say that the shows listed herein are unlikely to place on any list of the "best," no matter how long.
Bear in mind that this is just a list of shows that I'm actually familiar with. Undoubtedly there are tons of shows out there that have earned the "suck" distinction, but I just haven't personally come across them yet.
101 Dalmatians 9 to 5 The Addams Family Baby It's You Bare Big The Blue Flower The Boy From Oz Breakfast at Tiffany's Bring Back Birdie Carrie Come Fly Away Dance of the Vampires Dirty Dancing Dude Eating Raoul Evil Dead: The Musical Frankenstein Freckleface Strawberry Ghost Glory Days Good Vibrations Happy Days In My Life Into the Light Jekyll & Hyde The King of Hearts Kristina Legs Diamond Lestat LoveMusik Mamma Mia Mary Poppins Moby Dick Motown - The Musical Mutiny A Night With Janice Joplin Oh! Calcutta! The People in the Picture The Pirate Queen Rockabye Hamlet Rocky Roza Scandalous Sister Act Something's Afoot Soul Doctor Spider-Man: Turn Off the Dark Starlight Express A Tale of Two Cities Tarzan The Tin Pan Alley Rag Two Gentlemen of Verona Via Galactica Victor/Victoria Wonderland Young Frankenstein