Orson Welles went from Citizen Kane to voicing Transformers The Movie. I can't think of a sharper decline than that.
Transformers The Movie (probably) wasn't Citizen Kane's equal (I've never seen it) but TF:TM wasn't THAT bad and I thought he did a good job voicing Unicron. I will concede that it was probably more a "for the money than the art" kind of job however.