Which Opeth don't I recommend? Honestly though, Opeth is listen to the full album music. To this day, I struggle to listen to just one song as most of their albums are concept albums where one song feeds into the other.
If I had to list the best albums to start off with...hmm....
Blackwater Park
Still Life
Ghost Reveries
Watershed
Each is different...especially Watershed (their newest)
Tell ya what...drop me an email...I can hook you up with something that will help you get familiar with Opeth.
nabentroth at far.midco.net