for me has to be
Slayer's 1986 album "Reign in Blood"
it was the first album I ever bought, and it was also the same album that got me to listening to metal (next day I bought "Spreading the Disease" by Anthrax, "Peace Sells... But Who's Buying" from Megadeth and "Master of Puppets" by Metallica)
I still listen to it a lot, it's so hard to skip any songs, all are so good, and don't know where to start
"Raining Blood" and "Altars of Sacrifice" and "Angel of Death" are my most favorites on the album (in that order)
Long ago I used to be a fan of Grand Theft Auto game series, and when I played first with GTA Vice City, one of the first songs I heard on the game's radio station VRock was "Raining Blood" made me love the game just for it, discovered a few other bands along the way. By today I don't even own the consoles or play the games.