Opeth
Best: Advent
Picking a favorite Opeth song is pretty hard considering how much I like them. I picked Advent because I think it has more "great" riffs than any other song by this band, the clean vocals are especially haunting, and the accoustic stuff is really beautiful. Some others I was tempted to pick include: The Moor, When, Face of Melinda, Moonlapse Vertigo, and The Drapery Falls.
Worst: The Grand Conjuration
Still a decent song, I guess, but it comes off as far too monotone, and some of the riffs are almost directly ripped from Blackwater Park. I also considered picking Weakness from the Damnation album, but it doesn't exist enough to bother me. Dirge for November is saved by the opening and closing.
King Crimson
Best: Starless
Easy pick really. One of the greatest pieces of music ever, utterly beautiful. Some others worth noting are Red, Lizard, Lark's Tongues in Aspic 1-4, and The ConstruKction of Light.
Worst: Thela Hun Ginjeet
Alternately, pick almost any other song off Discipline. I really hate gibberish lyrics, and this album is chock full of them. Frame By Frame and the closing track save this album from being unlistenable. Some other KC songs that kinda suck are The World's My Oyster Soup... and ProzaKc Blues, both off the album The ConstruKction of Light (an oddity because all the songs there are either awesome or suck, there is really no middle ground).
Lugwig Van Beethoven
Best: 3rd Symphony (stretching the definition of a "song")
I'm a total Beethoven fanboy, but this is has long been my favorite piece of his.
Worst: I think some of his violin quartets are a bit forgettable, other than that, maybe his 8th symphony...
Godspeed You! Black Emperor
Best: Rockets Fall on Rocket Falls
Ahahah, this is like picking your favorite orgasm.
Worst: bbf3
I still like it quite a bit, but I feel it lacks the freedom of their other songs. Most times, the recorded speeches are included as something to add to the mood established by the music. In this case, it feels more like the music is included as a background for the ramblings of this crazy guy. I enjoy being able to let my mind roam in GYBE's songs, and I like the images I see in my head that the music helps to create. In this song, the images are already created for me. It's too tangible, and thus, less enjoyable for me.