Depends on your setup. If you're loking for crazy bass, check out Bass Mechanik, or 3 6 Mafia. Guys from the car audio boards always talk about Late Night Tip from one of the 3 6 Mafia CDs, but after buying that CD, the best track for bass on there (at least in my truck) is Ridin' Spinners. LMAO - I know that's some gay shit to bump around to though.
The 311 song, Transistor, metioned earlier hits HARD in my truck as well. I play it to show off the system to people who don't necessarily dig rap.
For LOUD rock, I suggest the very first song on the old (blue) Candlebox CD. I don't remember the name of it, but you can't miss it, lol. There are also several other good songs on that CD, by the way.
Tool, Rage, System of a Down, Sublime, 311, older Metallica, etc, just for rocking and playing it loud.
If you have a solid SQ setup, I recommend:
Phil Collins - Air in the Night
Eagles - Hotel California
Michael Jackson - Billy Jean, Dirty Diana, Man in the Mirror, State of Shock
Pearl Jam - anything from Ten
Jewel - several songs, I don't know the names, kinda gay but they can really display the abilities of your system
The list goes on and on. I know there are lots of other songs I like to play loud through my system, but I can't think of them all right now.