Ok, here's the rundown on Opeth with the exception of Orchid:
Morningrise - by far the most complete Opeth album albeit quite different to the rest. The vocals are more black metal than the death growls that come later and clean while the music utilises the classic dual melodic guitars that were prevalant at the time. The album consists of five songs, none coming in under 10 minutes, and all five are simply superb including my fav all time Opeth track 'The Ghost and the Silent Water'. A must have.
My Arms, Your Hearse - This is the album where Opeth changed styles a little and is more in keeping with what you've heard on Ghost Reveries. The production isn't perfect. In my opinion it's a fraction muffled but the songs are very good, particularly 'April Ethereal', 'When' and the classic 'Demon of the Fall'. A decent album
Still Life - Very similar in sound to MAYH but more consistent in terms of the tracks. Again, a little muffled but not a poor song on there. If I'm asked to list my favourite Opeth albums I tend to change a bit but Morningrise is always first and this is always in my top three. Best tracks are: Godhead's Lament and Moonlapse Vertigo.
Blackwater Park - I love this album. Some never warmed to it but the production is better than the previous two and the bass kicks arse, particularly on my fav two tracks 'The Drapery Falls' and 'Blackwater Park'. Mikael's death growls are at their most fercious and of course there are some moments of pure beauty. Will take a while to warm to but like every Opeth album it's worth the effort.
Deliverance - By far their best production on any album with the exception of Morningrise which simply oozes atmosphere. Very crisp sound although to me the album isn't as consistent all the way through. Deliverance and Master's Apprentices are the standouts. However, if you're an Opeth newcomer this might be a good starting point after Ghost Reveries. Fairly easy to get into.
Damnation - Throw all your expectations out the window. This is an acoustic album with all clean vocals. Very beautiful and an interesting aside to the progressive death they're renowned for. Think folk music with an Opeth touch. It's unmistakable Opeth but if you play it to someone they won't believe you when you tell them they're a death metal band. Standout tracks: Windowpane, To Rid the Disease, Ending Credits.
Anyway, my suggestion to you is to go out and buy ALL these albums even if it sends you broke. Also buy anything else I recommend to you at a later date

because I'm always right