Yeah, Reign in Blood is overrated. My least favorite out of their first 5. That's not saying it's bad by any means as I love their first 5. I do think a lot of people who don't really listen to it pay lip service to it just because it is supposedly "the greatest thrash album ever", though.
I have no one fav of the "Unholy Trinity". All three superb for different reasons. South Of Heaven for it being more "doomy".