I don't have a clue who Usher is, but anyone trying to redo what is arguably one of the best hoofing moments on film, and actually managing not to look like a damned fool, gets my respect.
Best cover song of all time? Depends on the genre, of course.
Anything Eva Cassidy does: Fields of Gold, Over the Rainbow, Amazing Grace, Bridge Over Troubled Water, Time After Time. That woman had an amazing voice, too bad she is gone.
Also, the entire Across the Universe has some good Beatles remixes.
I saw that Vanilla Fudge covered the Backstreet Boys' "I Want It That Way" and I thought "what the hell were they thinking?" but the made that song listenable.