What is the best iOS for iPhone 4?

What is the best iOS for iPhone 4?

Added (1). 32GB model.
Some Apps doesn't work anymore with old iOS.
How to restore (downgrade) some apps upgrade to work with old iOS

Best would be iOS 6. You can possibly downgrade using your shsh blobs which apple signs with for a particular device. Try searching on google and you should get a few options to downgrade. But you won't be able to do it using iTunes or any official way. Also downgrade won't be easy so I suggest that you make a backup before you try out anything, Do remember that many apps now require iOS 7 and soon it will be iOS 8