Live After Death (a double CD) is about all you need from Maiden (at first). It's got EVERYTHING. Otherwise, anything earlier than Fear of the Dark (when they started losing band members) is great.
For myself, I'm afraid I lost interest in Metallica after Enter Sandman. But MOP sure kicks butts.
Also, did you know that metallica is the species name for certain Austrailian ants? I wonder if it's because they are shiny? Or if it's because they run around banging their heads against the stage like they never did before, making it ring, making it bleed, making it really sore?