And I'd like to know where to buy the best hamburger.
Seriously... "best" is subjective. Everyone has an opinion, and those opinions may differ from yours.
Are you looking at a PC-based setup, a standalone setup (like TomTom or RoadMate) or a headunit-based setup by Pioneer or Kenwood?
If your answer is either of the latter choices, you're going to need to look elsewhere for opinions, since we eschew the pricey car audio products in favor of a complete PC in the car.