Any choice you make you will not regret in the least degree. They are all somewhat similar (they are all Opeth cd's after all) but also they are all they're own entities. Think of each cd a different project. Somewhat different theme, somewhat different mood, and different sound really.
Once you know all of them by heart you would be able to recognize which cd a certain passage comes from after 5 seconds of listening.
Whichever CD you happen to pick up, make sure to give it time. Let it sink in. Opeth grows on you, like fine wine.
Sfarog
oh. and by the way, I do not agree with the opinion that most older Opeth fans Dont like deliverance. I am willing to put my money on this: most opeth fans love Deliverance.