The answer is highly subjective and will depend on who is doing the evaluating. It is also doubtful that anyone has tested all of the offerings available in the app store. In a way it’s a moot point because infinite variety is not what we need. What we need/want is to find something that suits our style and vision.
About all any of us can do is to offer some speculation on the more popular offerings and hope we can shorten the search. Before I completely stick my foot in my mouth I should warn you that “some” people do not approve of filters. The reasons are many and varied.
I take this stance when it comes to editing photos. If you substantially alter the image it becomes what I call a “photo illustration” It’s becomes another form of expression and as long as you don’t try to pass one thing off for something else, it’s perfectly OK in my book.
I won’t attempt to espouse the virtues of each and every filter in each app but will supply a list of apps I like, having spent more than a few dollars on the price of my education.
Instagram – used for some subtle changes in light.
Camera+ [the one from taptaptap] More dramatic filter actions which are adjustable and may be combined. Also includes an extensive set of editing tools.
Hipstamatic – Takes a different approach by combining different films and lens to create a vast array of moods and styles.
Camera Awesome – Camera replacement app from the SmugMug team. Vast array of filters and editing tools as well as EXIF information about the image. Lots of in-app purchase opportunity. I like these filters for their rich tones.
Flickr [The App] Usually thought of as the photo upload site but recently they got into the app game with their own way to shoot, edit and upload photos to the web. Since people tend to think of Flickr for uploading photos it’s app is can be overlooked for it’s other skills such as shooting and editing. Fairly extensive set of filters and adjustment tools and one that shouldn’t be overlooked when looking for a good all in one editor.
There are lots of good apps out there; and this list is not to disparage the use of other apps or combinations.