Best app to edit photos "professionally" on an iPhone?

Snapseed, hands down. Miles ahead of the competition in UX and general ease-of-use, which is a regrettably low bar for the space.

When it comes to "beautifying" photos (messing with ANY contrast, saturation, vignettes, etc), Snapseed is my go-to. They also have an expansive catalogue of funky filters to layer and play with (make your pics look "old-timey", etc) that far outshadow VSCO or Afterlight (their closest competition IMO).

Don't know what it gives on iOS, but on Android it's the best one I've used so far

VSCO has been around a while, one of the first "professional" photo editing apps on the iPhone. Personally, the UX is confusing for me but many photo buffs swear by it.

I found VSCO and used it mostly because of its filters which are awesome, and whenever I tried another app I came back. Sometimes it makes the impression it's not for serious photography but whenever I use other apps, I miss the result I could get from it in a short time. I do wish for some changes like not having to import photos before editing, but after all the hustles and wishings, it's the app I use daily to process my photos.

I use Lightroom for iOS (for some reason no one has submitted the iOS app on PH) so have linked the Android version - but it's available in the App Store. It helps organise and edit photos in a pro way - and on the go! (plus Desktop version).

From the makers of Facetune, this is definitely one of the best photo editing apps out there.

Over is a beautiful app for adding text to your photos.

The Kardashians swear by this / use this on all their pics for a reason. IT FREAKING WORKS. It essentially gives you all the people-centric editing tools from Photoshop (think all the "morph" options and "red eye" fixes you can dream of). There is no reason you can't look like a total sexy babe in all of your Instagram shots, thanks to Facetune.

Besides VSCO which is quick way of getting things done and besides beautiful styles it provides (specially new filters on VSCO X), this one is my go to app for editing photos the way I used to process them on computer. Options and tools are like what adobe offers and with mobile friendly interface.