It's a tough choice.
I think the worst disaster concerned singers who tried to act because at least when actors attempt to sing you can ignore it more easily. I say that because you have to see the trailer of their new "masterpieces" and few years after you can still catch rerun of it on TV.
The best ?
Liza Minnelli in Cabaret and Cher in Mask.
The Worst ?
Madonna & Beyoncé.
The former has 2 or 3 "allright" stuff in her filmography while the rest is horrible but at least she eventually stopped.
The latter is probably one the most wooden and transparent actress I've ever seen. It's almost worse than bad because I don't feel anything watching her... Indiference.