I've heard from a lot of people that Navigon is the best so far. Although, TomTom just updated their iOS app to add weekly community map updates so it should have the most up to date maps continuously.
Personally, I'm waiting for Garmin to come out with an app or for Apple to update Maps to add text to speech navigation.
Currently I'm using MapQuest 4 Mobile and for being free it's pretty darn good.