The song that I have seen the people go nuts for when Opeth plays it is Deliverance. I think even though it is mellow for the most part, it gets the crowds booty shakin and a groovin.
My favorite song for a while was "Baying of the Hounds", until I was really stoned one night on some killer Hydro Bud, and sat back and enjoyed "Beneath the Mire". I love the way that the song "builds" from very mellow, to open and airy, to heavy to psycho heavy, back to melodic and structured, to a laid back jazz and piano groove, then to a whimsical guitar part, to heavy death growl, back to structured clean driving metal, then to an open airy guitar rif, to heavy pounding death metal, to a killer staccato drum ending fading off in to the distance...... bad ass. Wish I could conceive of music like that.