I think the art of a good cover is to make it different enough from the original, preferably a completely different style, but still be as good. This means that the better the original the harder it is to do a good cover. Anyway, here's some of my favourites, some fit the above, others are pure parody, others are both.
This next one came about as the music press claimed Oasis' Shakermaker ripped off the riffs from I'd Like To Teach The World To Sing. No Way Sis were the only Oasis tribute band to be approved of by Oasis themselves.
This next one is awesome, if Stairway had been a Doors track, this is what it would have sounded like:
This next one fits my first criteria, Don't Fear The Reaper has to be the classic slow rock song. Apollo 440 turn it into a dance track and do it well.
One more for now, a nice ska version of a top Lita Ford number: