Top 10 Most Intricately Detailed Metal Album Covers

The Top Ten

1 Somewhere in Time - Iron Maiden

It contains many references to the band history, science fiction stories, movies, previous Maiden sleeves and songs, band members favorite things, events, and in-jokes.
There's even a hidden backwards message that reads "THIS IS A VERY BORING PAINTING".
There are references to: their songs 2 Minutes to Midnight (clock reads 23:58), Acacia Avenue (Eddie's standing on Acacia Avenue), Aces High (Aces High Bar), Phantom of the Opera (Phantom Opera House), Rime of the Ancient Mariner (Ancient Mariner Seafood Restaurant); Margaret Thatcher (Maggie's revenge), their favorite soccer team (Latest Results... West Ham 7, Arsenal 3), Steve Harris' first band (Tonight GYPSY'S KISS), one of the band's favorite bars in the US (Hammerjacks), drummer Nicko McBrain is wearing a shirt that reads "Iron Wha? ", and many more.

My favourite album cover by far

2 Live - Blind Guardian

You see only the front cover. But there's a back cover art, too. Same style.
"Live" is a live album and the "Bard's Tavern" is where the bards from Blind Guardian play live. You can even see one of the bards through the tavern window. "Bard's Tavern" is a reference to their song "The Bard's Song".
All details are well visible, including the small streets, staircase in the end of the street and its steps and handrail, lights in the houses, the dragon's tale next to the tavern entrance, clients in the tavern, etc.
Cover Art - Andreas Marshall (master of details - there are more arts by him on this list)

3 Gutter Ballet - Savatage