The one cover which really bugs me is "Umbrella" because it's part of a trend I really hate - male guitar bands covering great pop tracks by women. It always comes with a whiff of irony and sexism which riles me. There is no way in which the Manics' "Umbrella" is better than Rihanna's original, which is an absolutely perfect pop song. I mean, I enjoy their version, because it's a competent take on a great song, but if I want to listen to "Umbrella" I'm absolutely going to listen to Rihanna, not the Manics.
Sorry, touched a nerve, rant over
As for their other covers I have no real opinion as I don't really listen to them much - apart from "Out of Time" which I adore, but am not that familiar with the original.
Great idea for a thread though! And now I've been reminded how much I love the original "Umbrella" I'm off to watch the video and reminisce about that summer of 2007 when it never stopped raining...