Slayer are one of my all time favourites and I love all their music. But if I could only keep one it would have to be Reign in Blood. I bought this as soon as it came out and I never tire of hearing it.
"Reign In Blood" would be the popular pick, but my personal fave is "Show No Mercy" since I started from the beginning, and it was my first Slayer album, so sentimentally it's my fave.