I can't say which is the best because I've only used one. The app that I use is the GuitarToolKit. The app is 9.99 but it's well worth the price. IMO it's the most clean looking one in the app store. It works exactly like a chromatic tuner, plus it comes with extras like a metronome, guitar chord finder, and it has tones to help you tune by ear. I think the tuner itself justifies the 9.99 price tag.
If anyone else uses the same app or the other tuners, what are your opinions?