Really depends on how crazy you want to go. I didn't want to do the single-DIN because it was going to cover up the A/C controls and the motorized parts of the unit tend to break.

Kenwood uses the Garmin software with Navteq maps but has less POI than the Pioneer
Pioneer uses TeleAtlas maps, not a good routing, but nearly double the POI as Kenwood.
Alpine uses Navteq maps but not sure of the Nav interface.

I liked the Pioneer the best but single-DIN units have a separate brain but double's sometimes have HDDs.

Post up what you're looking for in the unit other than just Navigation.