What's the best free photo editing app for iPhone?

I'm doing an editing contest on Instagram and I don't have any good apps and I need help!'

Photoshop app

Snapseed is excellent, probably the best free app. Photoshop's PS Express is good also.

Instagram is definitely the most popular. However, you should look into other cool ones such as:

Snapbucket
Picture Effect Magic
Tattoo Shop Lite