Sodom covering Ace of Spades was fucking rocking, cause Sodom is basically Evil German Motorhead On Crystal Meth. Heathen did a really badass cover of Queen's "Death On Two Legs." And I have to mention Overkill's cover of DOA's "Fuck You," because you wanna talk about making a song your own...
Oh yeah, Blue Öyster Cult's version of "We Gotta Get Out of This Place."
Worst is easily Cradle of Filth's absolute MURDER of "Hallowed Be Thy Name." Also, as much as I like Forbidden, they fucking tortured "Victim of Changes" on that live EP.