[Suzerain of Sheol]

My longest is Jethro Tull's Thick as a Brick, which is actually two 22-minute songs, but they're only split because of lack of space on the record when it was originally released, it's actually one song of 45 minutes. But it changes a lot throughout, so it doesn't get boring.

My favorite long song, though, is Death Come Near Me, by Draconian, 17 minutes of soul-freezing despair and anguish, slow and devastating. Perfect for sunny days. :p