The question is, do you want rap songs with hot basslines/beats or shit will just knock hella hard?
I'm an old school rap fan, personally. Whether it's hardcore gansta rap like NWA or cheesy shit like Tag Team, that is my thing. Apart from NERD and Outkast, I haven't heard a lot of newer rap acts I like. I burned a rap CD for my car, here are some of my favs (out of a few hundred on an MP3 disc)
NWA ft Easy E - Boys In Tha Hood
Digital Underground - Humpty Hump Dance (K i know you'll probably look past this one and think its too gay to be pounding in a car, but the bass groove is the coolest thing ever)
Dr. Dre - Let's Get High
Dr. Dre - Bang Bang (I think it's safe to say all Dre is awesome)
Sir Mix-A-Lot - Baby Got Back
Jim Crow - Holla At A Playa
Methods Of Mayhem - Get Naked
Big Moe - Purple Stuff
Dunno. My rap knowledge is limited. Only got a couple hundred songs.