That is very tough....... however, two stand out as far as "bigger" songs that were covered........
Alien Nation - Avalon.
Egypt(The Chains Are On) - Doro.
Bryant
I agree both are really well done covers.
Wardog - 'Purgatory' (Iron Maiden)
Demonios - 'Fear Of The Dark' (Iron Maiden)
Testament - 'Rapid Fire' (Judas Priest)
Overkill - 'Tyrant' (Judas Priest)
Skyclad - 'Dreamer Deceiver' (Judas Priest)
Steve Howe & Annie Haslam - 'Turn Of The Century' (Yes)
Gregoor van der Loo/Marcel Coenen/Trent Gardner/Jeff Brockman/Carl Cadden-James - 'Freewill' (Rush)
Ugly Kid Joe - 'N.I.B.' (Black Sabbath)
Sonata Arctica - 'Still Loving You' (Scorpions)
Tim Owens/Yngwie Malmsteen/Tim Bogert/Tommy Aldridge/Derek Sherinian - 'Mr. Crowley' (Ozzy Osbourne)
Lisa Loeb/Dweezil Zappa/Michael Porcaro/Stephen Ferrone/Micheal Sherwood - 'Goodbye to Romance' (Ozzy Osbourne)
Solitude Aeturnus - 'No More Tears' (Ozzy Osbourne)
Therion - 'Southbound' (Thin Lizzy)
Ronnie James Dio/Yngwie Malmsteen/Stu Hamm/Greg Bissonette/Paul Taylor - 'Dream On' (Aerosmith)
Sargant Fury - 'Eagle' (ABBA)
Rough Silk - 'Take A Chance On Me' (ABBA)
Steel Prophet - 'Ides Of March/Purgatory' (Iron Maiden)
Solitude Aeturnus - 'Hallowed Be Thy Name' (Iron Maiden)