I had Metallica and a couple of other metal bands on my Ipod before, but for me Opeth were THE band that got me into the more extreme realms of metal, more specifically The Moor. Haven't got a weak album IMO, excellent song writers.
However, I don't think there is a trio of albums that are better overall than My Arms, The Moor and Blackwater Park. Their music is jaw droppingly brilliant.
Blackwater Park is my favourite metal album to this day.