I think the thread should be limited to people who sing as their day job, not sports stars or actors like William Shatner. Alright, bad example, not an actor either.
Anyone can be persuaded by a sycophant hanger on, that the world has been waiting for their off key crooning.
Some wondrful suggestions, not many classical. May I nominate Monserrat Caballe, the Spainish Soprano. One of those voices that go through your ears like an icepick. An icepick would be preferable.