Reign in Blood rules. It is one of the greatest thrash albums of all time, and imo is Slayer's best if you're looking for crushing speedy thrash. I think Seasons is much more accessible, but I prefer the more speedy thrash of RIB.
Some of his fills on South Of Heaven are Insane. But as much as I respect them for introducing me to more types of metal, Slayer themselves are totally overrated. I love them, but they are. Reign In Blood was good, but it wasn't their best album. I prefer Seasons and SOH myself. Reign In Blod was good for the first few songs, then it felt like one big song...Thats where I find Slayer to fall down, their is little or no variation in their songs, it's either a clean, creepy riff, or a crazy thrashy one. And I'm sorry to say it, but KK isn't worthy of the guitar he holds. Jeff Hanneman however..there's a different story..
Postmortem and Piece by Piece are my favorites from the album. Personally, Ill stick with Seasons in the Abyss as best Slayer album. Due to Blood Red.