I'd say "The Great Annihilator" is a good place to start, it has "airwaves format" songs you can get into straight away and become acquainted with the band's personality before delving into their more experimental/droning side, whereby "Soundtracks for the Blind" is their best album IMO.
This said, you might want to get "Various Failures" first since it's about the only release that's still available at <$50 and it encompasses a good summary of their career, including fantastic songs from the long OOP "White Light from the Mouth of Inifinity", which I think is NAD's favourite if that says anything.