Screenager is insanely underrated. It has brilliant guitar work, nice bass and Dom actually used animal bones on the song. Plus, I love Matt's vocals on it, they're so beautiful, as well as utilizing the lovely falsetto that he used in Muse's earlier days. I also love how Screenager is pretty and unnerving at the same time. Screenager is another high point on the magnificent masterpiece that is Origin of Symmetry. I'm kind of surprised that Screenager doesn't get much love from fans.
Additionally, Falling Away with You, Falling Down, Soldier's Poem, Hoodoo and Guiding Light() are underrated.