If you could choose one song to some up Opeth...

Can't chose one, it would have to be a song with a great mix of their soft/heavy/progressive/melodic styles.

Something like When, The Moor, The Drapery Falls, Deliverance, Ghost of Perdition.

One off each of their more important albums there.

I think either the Moor or Ghost of Perdition sum them up the best.
 
patterns in the ivy pt. 2

no growls or distortion, i know. but i think its their best song, and thats the song i think anyone with a decent appreciation of music could enjoy.

as far as opeth's conventional songs, id say:

when
baying of the hounds
dirge for november