I can't stress enough, be sure you listen before you buy, but these are the brands that I have personally listened to lately that sounded good to me:
<<Opinion mode ON>>
ScanSpeak (believe it or not)
In that specific order.
These systems were in competition quality vehicles, and/or manufacturer demo vehicles, so the signal source and power provided to these systems was likely to be as optimal as you might get to in a car.
<<Opinion mode OFF>>
The DLS are likely to be your best bang for the buck, as you can find them relatively cheap, and their sound is in the top three in the world.