what is the best camera app ?

That really depends on what YOU want. If you want an extremely versatile camera app, Open Camera is a great option. Although the UI isn't exactly pretty. If you're willing to pay, ProShot is pretty excellent. But what kind of camera experience are you looking for ?
 
As Baz says, there is no "best". However, with a list of features you want, we can point you in a good direction.

What phone do you have?
Which version of android?
Which carrier?

Which camera app are you currently using?
Which features are you looking to improve on?