It's fortunate that I was introduced to Opeth by a friend who had the sense not to try and classify them. If she'd said "black metal" or "death metal" I might have been less inclined to listen. Fortunately she didn't, and I'm now the proud owner of all the albums by this phenomenal group.
Being relatively new to Opeth, I couldn't say if there is a "best" album, but Blackwater Park is a superb one without any question. The sheer amount of great riffs, melodies, beats and vocals (yes, he does growl, but when he sings clean, he has a wonderful voice) is quite amazing. Unlike many bands, for whom the idea of contrasting loud and soft is nothing more than a gimmick, the heavy stuff here is as good as any metal band you could name, and the soft stuff is genuinely gorgeous to listen to. The acoustic "Harvest" is a great song, and the guitar lines on "Bleak" are inventive, powerful and mesmerising. The title track itself is another standout track, flowing from soft to hard with no sense of difficulty at all. But really there is so much on offer here that it is pointless to single out individual tracks.
Opeth will, if there is any justice, gain a reputation as one of the most important bands in recent memory. In terms of variety, musical skill and inventiveness there are few around who can get close.