Madonna's cover of Santa Baby is one of the worst things ever.
Santa Baby is a decent Christmas song tailor-made for Eartha Kitt and her delivery sells it and makes it great.
Madonna completely misunderstands the entire tone of the song and what it's about and turns it into this retarded puerile Betty Boop thing that makes it just as, if not more, dumb than every other Christmas song ever made. It is the opposite of fun and the opposite of sexy, and I hate it.
I've also just heard tale of a William Shatner version of Common People which has been described to me as a travesty. Suffice it to say that I haven't heard this version, but I'm assuming that Shatner doesn't really get whatever humour is actually already in this masterpiece of a song, and is just pooping on something good in total ignorance. Sure, he's in on the "William Shatner is a shit singer" joke, but he's not in on the "William Shatner as a shit singer is a shit joke" truth.