I envy those who actually can pick just 1 single song per album. Well, here's my try...
Orchid: The Twilight Is My Robe - The bass solo, the very long instrumental parts... This song rocks like hell, hehe
Morningrise: Nectar - It's the first song Opeth I ever heard and still every time I'm impressed by the end of this song. The jazzy instrumental part in 'The Lotus Eater' does sound a bit like the Nectar end actually.
My Arms, Your Hearse: Karma - Sorry, but this is my least favourite album. Karma is one of the few songs I like.
Still Life: White Cluster - Oh hell, this is hard. It could so easily be 'Godhead's Lament' or 'Moonlapse Vertigo'. But the goosebumps solo in White Cluster gives it a little advantage.
Blackwater Park: Blackwater Park - Not very hard to choose, this song is one of the best ever made. The end again is marvellous, all credits to Mikael for the "Sick liaisons raised this monumental mark. The sun sets forever over Blackwater Park" phrase.
Deliverance: Deliverance - Absolutely the best Opeth song until the end of days.
Damnation: To Rid The Disease - Lyrics + music = perfection
Ghost Reveries: Atonement - Very relaxed song, with an incredible masterpiece at the end of the song.
Watershed: Hessian Peel - A potential Deliverence, no doubt