Tom Tom uses inferior map sourcing called TeleAtlas or TeleNav or something. Navigon (and Garmin) use NAVTEQ the leader is mapping. Navigon is also a German company which alone makes it the best.
I don't have enough experience with any other Nav apps to be sure, but I do enjoy Navigon, and it is not the best, it ain't nowhere near the worst. It is the most costly iOS app I own, but I enjoy using it.