Not sure if the best, but sure a giant step in their career as they become bigger and bigger after this release. Definitely, one of the most importants albums in Thrash Metal history and Metal in general. A pleasant storm of speed!
South Of Heaven is great, and they've had a lot of great albums since. Seasons In The Abyss, Diablous In Musica, God Hates Us All, Christ Illusion, Repentless, all have been great. Honestly, for me personally, Slayer is the one band out of the big 4 that has retained my interest after their initial "classics". Megadeth has had a couple albums here and there and I enjoyed their new album Dystopia, but it's been very inconsistent. Metallica.... we don't talk about Metallica. Anthrax has been all over the place for me, although I really dig their past two releases ("For All Kings" is absolutely great). Slayer is the one who's remained consistent, at least in my opinion. Out of their three major "classics", I think I take Seasons In The Abyss.
In 1986 - hell! Today - overrated. No one really seems to give a shit about this album apart from the two obvious songs, but on the other hand you cannot really make a full album of songs with that caliber. It's good for what it is, I suppose, but it sure was a lot greater for its time. Not their best but certainly essential for any fan of metal.
The most aggressive, concise, relentless, speedy thrash album. Listening to it loudly always feels like smoking a bowl of speed. It stands the test of time as one of the best albums of all time. This is a 3-Way tie with South of Heaven and Seasons in the Abyss as a Slayer favorite album for me.