I highly recommend
Navigon MobileNavigator in either the full or regional version. It blows away my old standalone Magellan and my parents' TomTom by wide margins.
I much prefer the way Navigon displays routes and highway signs and tells you which lane(s) to be in. And the live traffic add-on, which is an in-app purchase, is really helpful, especially living and driving in the traffic hell that is Southern California. It has full text-to-speech (meaning it tells you to turn on Main street, not just to turn in 100 feet.) That means you don't have to take your eyes off the road if there are short blocks.
As for window mounting, I have a Griffin WindowSeat, and it was great for my 3GS, but it doesn't fit the 4, so I've been keeping it in a cupholder in the center console, and the GPS can still read my location just fine.
🙂
I also like the little touches, like the fact that you can tell it whether to pause or fade the audio you're playing while it gives you directions. If I'm listening to music, I tend to have it fade down, but for podcasts, the automatic pause feature is great.
