As a general rule, cover versions are virtually always a lot, lot worse than the original.
I'm aware of two exceptions where the artist has improved the original. In both instances, they have had creative input to make significant changes to the song, such that you could almost give them a writing credit.
The first is Jimi Hendrix's cover of Dylan's All Along The Watchtower.
The original is of course a great song but I think it's fair to say that it's more about the bare bones (the chord structure, melody and words) without the decoration - which Hendrix added with aplomb. He also had a great drummer in Mitch Mitchell.
The second one is Lost In Music.
The original by Chic / Sister Sledge has some incredible musicianship from Bernard Edwards of course, but it also has some of the worst lyrics that I've ever heard, such that it's hard to listen without cringing.
The Fall took the best bits of the song, took out the awful verses and added a lot of their own: Mark E Smith rewrote the majority of the lyrics; the band added some fantastic keyboard and guitar parts and generally gave the song a different feel.