The one that immediately comes to mind is Godfather Part III. Why? Sofia ****ing Coppola.
While it wouldn't have been as good as the first two, with a real actress (IIRC Winona Ryder was originally cast as Mary), it would have been infinitely better. Coppola absolutely nuked every scene she was in.
Another one was the musical Les Miserables. Why? Russell Crowe trying to sing. Holy shit - what were they thinking? Everybody had to realize how awful he was, including him. I guess he must have needed the money, but Christ, did they sign him before hearing his voice? He basically mumbled the lyrics of everything he sang. Absolutely awful. Really pissed me off because I love the stage production so much.