Depends entirely on my mood.
If I'm feeling angstful, then some good ol' fashioned heavy metal / hard rock.... [pre-1996] Metallica, Slayer, Iron Maiden...
If I'm feeling peaceful (like when it rains), then my absolute all-time favorite singer: Sarah McLachlan. Also usually throw in some Sheryl Crow, Dido, Norah Jones, etc.
If I'm in a general good mood: Joe Satriani (makes for GREAT driving music), classic rock like ZZ Top or Lynard Skynard...
When I get sick of the same old crap being repeated on the radio stations, I flip to the hip-hop channels and there's a couple artists I actually find entertaining... Eminem, Ludacris, Jay-Z, etc.
Right now, the 6 CDs living in the changer are:
Sarah McLachlan - Afterglow
Sarah McLachlan - Mirrorball
Joe Satriani - Live in San Francisco disc 2
Motogrator - Motogrator (pretty heavy stuff)
Union Underground - Union Underground
Shania Twain - Greatest Hits