Black Water Park and Morningrise were the two that started me off, but as everyone else says, get em all and now! you wont regret it. hmn, it makes me think, an opeth box set would be a great thing to make , oh well i can always dream.
This question comes up every once and a while and I always end up thinking the same thing: it really does not matter what order you get them in. All five albums are masterpieces and you will eventually end up getting them all anyway(I think Opeth is the only band I can say that about ). So I say just get them in whatever order you can find them. But I wouldn't recomend getting more than one album at a time, since it takes a while to fully absorb an Opeth cd and its alot easier if you take them one a time.