I also think Blackwater Park is good for a starter.

my arms your hearse, and blackwater park

you so have to get those, or else ill haunt you till the end of DAYS

Ghost Reveries is awful.

Best album to start with is probably Blackwater Park. Still Life is the best album ever made.

Oh and keep in mind that their first two albums, Orchid and Morningrise, are quite different in style from the later ones.

The Roundhouse Tapes is excellent too, but it's a live album. Good blend of songs from all albums though, might be good to get acquainted with the 'Peth.

I started with Blackwater Park and it was a great introduction.

I'm kinda torn when it comes to Ghost Reveries. I haven't listened to it in like half a year but it was my favourite album like 2-3 months after I first heard Opeth. I can't stand it right now because of the production and overused synth, but the songs themselves are great. Awesome live though. Actually, Opeth are awesome live no matter what they play. Btw, Portnoy included the concert where Mikael sings the first minute of Repentance @ Prog Nation on Chaos in Motion, if anyone cares.