I have Foreflight, WingX and AirNav Pro...
The choice of which depends largely on the map set I want to use.
For Europe and Africa AirNav I find better.
For the US I find it a toss up between Foreflight and WingX, with a bias to Foreflight from an ease of use perspective.
There is a thread here somewhere, I think it is called iPad apps, where folks have offered comments....