Here's a few of my faves/hates:

Best:
Savatage:Gutter Ballet (awesome,surprised it hasn't got a mention so far)
Van Halen (1984,5150,Balance)
Ozzy Osborne (Ultimate Sin)
Dream Theater (Images and Words)
Fair Warning:s/t
Hurricane:Slave to the Thrill
Lillian Axe:Love and War (what a babe!)
Lillian Axe:Poetic Justice (good for a laugh)
Killer Dwarfs:s/t (cheapest cover of all time,but gets a laugh)
Elegy:Lost
Bangalore Choir:On Target (again for the girl)
Stratovarius:Fourth Dimension
Red Dawn:Never Say Surrender.................................I could go on........

Thought I would be mostly positive,but top of my list for worst covers are Journey(Trial By Fire),Van Halen(Diver Down),Glory(Positive Buoyant),BAd Moon Rising(Opium for the Masses),Big Bad Wolf(s/t),......................again I could keep going.