The first Opeth song I ever heard was Bleak, and at first I thought it was a little strange, but it definately grew on me and I thought it was one of the greatest songs I've ever heard.
Then I got Blackwater Park and I was blown away. But not at first. The first 2 or 3 times I listened to the album, for some reason I would always fall asleep through it. Perhaps it was because at that time I wasn't used to lengthy progressive music, because I was going through my 2-3 minute metal blast stage, Napalm Death, early Lamb of God and such.
Then, The Drapery Falls: the song that has blown me away more than any other song I've ever heard in my life. The first few times I listened to it, I got intense chills. And from then on I was sold, and became obsessed with Opeth.
Now I have all of their albums except for Still Life. And I am actually considering getting a tatoo of the Opeth O on my back.