There are so many apps for editing and adding special effects that it's pretty hard to keep track of them all. I've reviewed so many that they can become a misty blur. Effects Studio (US$0.99) takes adding effects to your photos way beyond where most apps go but still manages to offer the basics, so it does stand out from the crowd.
You get the expected filters, including all sorts of wild colors and polarization effects, as well as common adjustments of luminosity, color and contrast. There are focus and blur tools, as well as tone and curve tools.
Where Effects Studio steps up with something beyond the basics is additions like horror masks and hair pieces. Now I realize most people won't want to add grotesque teeth to their photos of family and friends, but Effects Studio does this in such a way that it doesn't look like something pasted on a photo. You can re-size and blend the effect onto your image, so it looks as 'natural' as something like this can look.
There are 30 frames to choose from, and most generate masks so the frames can be more ornate than a simple rectangle. A resize command lets you change the size of your photo to any of several pre-sets, or to a custom size that can be upscaled or downscaled from your original. There are brushes for adding to your photo, in addition to text and cropping tools.
Effects Studio seems to work both ends of the editing spectrum. There are pro tools like curves, while there are also some wild effects that can make your images pretty bizarre.
Effects Studio is a nice package for $0.99, and it only requires iOS 6 or later. It's not a universal app, and it should be. It will work on any iOS device, but not at native screen resolution on iPads.