Among each album they would be IMO:
Showbiz: Hate This and I'll Love You
Orgin of Symmetry: Dark Shines (I don't really think there is an underrated song on this album)
Absolution: Endlessly and Sing For Absolution (GREAT song)
Black Holes and Revelations: Hoodoo
The Resistance: Guiding Light (I think this has killer vocals, a GREAT freaking guitar and some great drums and bass as well, screw the cheese this is a great song)
And as for B-sides i'd say Glorious i there best but their most underrated would be Shine (both version but I especially love the acoustic version) or Hyper Chondriac Music