Beneath the Mire would be impossible because of the solo.
Honestly though I would say The Drapery Falls. The chords in the beginning are fun to play on guitar. IDK about guitar hero though. All I know is that a regular guitar > guitar hero.
That song is actually going to be in Guitar Hero III, together with "Number of the Beast" and more. The interesting thing is that they try to use the original recordings this time, not covers. Will be out for PS2/PS3/360 and Wii.