I'd represent Blackwater Park, Still Life or Ghost Reveries next. GR is probably the closest in style to Watershed, but BWP and SL are often considered Opeth's creative pinnacle and are a must-listen for anyone that likes music in this style. Really, unless you don't pick up one of the first two albums (though they are pretty awesome as well, but they did take a long time to click) or Heritage (same thing, though it doesn't really have much in common with the classic Opeth sound), you'll be fine, though. Deliverance isn't an album that I'm quite fond of, but most people like it a lot, so don't take my word there.