Most Influential Extreme Metal Albums

The Top Ten
1 Black Metal - Venom

It started a new genre

2 Scream Bloody Gore - Death
3 Morbid Tales - Celtic Frost
4 Under the Sign of the Black Mark - Bathory
5 Scum - Napalm Death
6 Seven Churches - Possessed

First death metal album.

7 Ride the Lightning - Metallica
8 De Mysteriis Dom Sathanas - Mayhem
9 Welcome to Hell - Venom
10 Reign In Blood - Slayer

This has my vote.

The Contenders
11 Kill 'em All - Metallica

Not the best Metallica album (still great though) but I think it was their most influential album because it started thrash popularization.

12 Blood Fire Death - Bathory
13 Gothic - Paradise Lost

Cambio la musica para siempre

14 Vulgar Display of Power - Pantera
15 Roots - Sepultura
16 Alice In Hell - Annihilator

Influenced Megadeth's Rust in Peace and their next recordings - as a result Megadeth stuff became even more technical, especially guitar work. Alice In Hell and Jeff Waters impressed Dave Mustaine - I guess it's not easy to impress him with guitar skills but Jeff Waters did.
For Rust in Peace Mustaine wanted somebody like Jeff Waters on the lead guitar and because Jeff declined to join Megadeth, Dave hired Marty. And Marty was an incredible technical guitarist, too.
Actually not only Megadeth but also the stuff of the other thrash bands became more sophisticated in the early 90s thanks to Alice in Hell. Competition works, haha.

Some proof that it influenced Mustaine? Some evidence to back those assertions?

17 Don't Break the Oath - Mercyful Fate
18 Bathory - Bathory
19 Pleasure to Kill - Kreator
20 Obsessed by Cruelty - Sodom
21 Theatre of Tragedy - Theatre of Tragedy
BAdd New Item