It depends on what I just installed and what I'm looking for.
If I want all around sound, I usually go for Staind - Outside as the recording is very well done and has all sorts of different sounds to it. If I'm looking for bass, I head to either Dawn of the living Boom from IBP or Bass Mechanik's Toccatta in B / Pump up. If I'm looking for nice midbass, I'll play Blue October's Into the Ocean. For nice mids and highs, I go with about anything from Dave Matthews.