Slacker is best because of...
Caching. I work as a rover in a pretty rural area, very little coverage while driving between towns, and as there is no good FM radio, I maintain my sanity with Slacker. However, as I am somewhat picky with my music, having a subscription would let me ban and skip many songs allowing me to fine tune my stations that much faster!
Along with that is the G2. I have a G2, and like it very much! Even though I have the app on my R2-D2, the G2 is very handy! And it holds a ton of music!