Depends what your need is, for turn by turn verbal directions I've used the at&t navigator app I also have on my iphone, on my ipad. But otherwise my favorite is just the maps app. It's not perfect, but I've yet to use a GPS app that is, but I like that you can actually view the map while getting directions, instead of the having the whole page taken over with text directions like some apps.
I've used a couple others and didn't like them.