The best? Sygic Aura!
I have Sygic Aura BeNeLux and it's a really nice GPS, better than TomTom or Ndrive (these two are the ones I have tested already, can't tell you more about other GPS software for the iPhone).
Have a look at the
Sygic website for more info and watch some vids on YouTube or read some reviews: you really want this GPS and nothing else.
Also, you can use it to go for a walk as well, it's very nice to walk through Brussels with 3D buildings around me