I love old hip hop mixtapes and blends.
...and anything that you find on WEFUNK Radio - Hip-Hop & The Original Funk
...and then I'll also play Dredge, Emery, Sunny Day Real Estate, Tool, Faith No More, Radiohead, Alice In Chains, 311, Nirvana, Daft Punk, The Invisibl Skratch Piklz, and Fugazi.
Put all that (and some classical organ music) on a USB drive and set the play function to "random", and that's what I'm listening to in the car.
I also like ELP (Emerson, Lake, and Palmer), Yes, Frank Zappa, Led Zeppelin, classical, and jazz, but I like to play FLAC versions of those so they are listened to at home.