Why not take them in chronological order? First Orchid, Still Life and finally Blackwater Park. All three records are amazing, so it doesn´t really matter which one you buy first.
personally, i got Opeth's catalogue in the following order:
Orchid
found out how great it is, and ordered the rest of the catalogue (that was before BWP was out) - Morningrise, MAYH and Still Life, probably my best Amazon.com order ever
and then BWP
np: Eifo Hayeled - Haviu Et Hastav (if you swedes, fins and nords can listen to your local music, i can listen to mine!)