What formerly great actors (actresses) have degenerated to so-so or just plain bad?

In my opinion not many performances compare to Al Pacino as Michael Corleone in the first and second Godfather movies. As time moves forward, Pacino’s skills seem to be moving backward. Slowly but surely his performances have degenerated to the point where he is parodying himself and chewing the scenery in movies such as Jack and Jill. Can you think of any other once powerful performers who have suffered the same fate?