I used to transmit through an FM transmitter. I found that even the best quality out there was not good enough so these days I use an mp3 reading cd player in my car and can get almost 200 songs on a CD.
I even tried a DVD capable one since DVDs can hold about 1,000 mp3s but I wasn't happy with the unit I found.
So my personal recommendation is the mp3 cd route.
Anywho, back to what you actually asked for:
When I used an FM transmitter I found out that the allowable signal output is limited due to regulatory bodies and that I could boost my signal merely by extending the cord that plugged into my device (my transmitter used that cord to transmit).
Because of signal changes when you move around you might need several dead spots preset on your car's tuner and you may often need slight adjustments.
Because of this an FM transmitter that has digital tuning and multiple presets is a lifesaver. And the ones that kept winning when I was spec'ing my own FM transmitters were the Belking models like this one:
Belkin FM Transmitter
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