You might as well ask a group of interior decorators what the best color is. You'll probably get as many different answers.
My preference? JVC

As an IASCA Sound Quality Judge, I have heard many systems with many different brands of head units. I can tell you based on that experience that what happens to the signal after it leaves the deck has a much greater potential to degrade the sound than selecting one brand name over another. However, all else being equal the specs I personally look for are a signal-to-noise ratio greater than 100dB and a Dynamic range of greater than 95dB. I look at these mostly because the audio chain is only as good as its weakest link, and performance at this level will keep the CD player on par with most better-quality amps and processors.
Beyond that it is all personal preference on features and appearance.
In case you're wondering, the JVC Digifine 2.1 series has a S/N of 102dB and Dynamic Range of 98dB.
