It depends what type of riding/mapping you want to do.
I've tried a bunch, but the three I use regularly are:
* the built in maps app
* google's map website in the browser (which you can save as a bookmark to the home screen)
* MotionX GPS (not to be confused with MotionX Drive, a different GPS app by the same company).
All four are useful for different situations. How do you want to use it?
The built in haps app has really good turn by turn instructions. You just speak where you want to go, and it reads out each intersection as you get to it.
Google's map website is great for finding local business listings (all four have this feature but google does it best)
Sygic has the best street map in my country - far better than the others - great for navigating remote areas of Australia. It is also fully functional without any phone coverage.
MotionX is the best one for trail riding and bushwalking - topographic maps and the like. It also has similar features to a Spot device, if you want to share your ride with friends/family.