"Best" is kind of subjective. What's best for me, might not be best for you.
With that said, I tried a number of them after I converted from Android (Google has an excellent GPS app on Android, that they haven't ported to iOS). I found the cheap ones like MotionX GPS Drive and CoPilot to be lacking. I eventually settled on Garmin's GPS app and found it to be the best of the bunch and at a relatively decent price.