I think that Sabbath Bloody Sabbath cover is pretty bog standard really. The version by Bruce Dickinson with Godspeed on the Nativity In Black tribute album, on the other hand,wow. When Brucie yells "You bastard" it sounds so good.
I haven't heard all the covers, although I'm listening to a compilation of them that I guy I met through e-bay sent me at the mo.
Auf Wiedershien, London, Celebrated Summer, She, Love Her All I Can,
They're all good
Probably my two favourites are Milk and Got The TIme, Got The time was one of the first songs that really got me into the mighty 'Thrax