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