Which sounds best is a loaded question... head units are more about features than sound quality of the CD transport... but features like crossovers, DSP, and EQ can change the sound.
Add-on HD tuners which plug into the head unit would give you the best AM/FM quality. Analog tuners in Kenwood, Eclipse, Alpine, and Pioneer sound better than others, (such as Sony, for example), with pioneer being the best. I'm looking at buying another deck for another vehicle right now and I'm looking at Pioneer for the analog tuner and the DSP crossovers which appear to be very good... there are so many features though, it really depends on what's important to you.
If you ever thought about trying surround sound, panasonic has a few decks with CS automotive.