I had been a VSCOcam user since launch, but I've kind of grown tired of their recent UI changes and wanted to try something different. I really like how easy Darkroom is to use. My favorite feature is being able to save filter edits as a template for future photos since I pretty much only use one or two filters for all of my photos that I post to Instagram.
It would have to be Instagram. I try to limit the time I spend on mobile, but whenever I do, I always like browsing through the pics and there are so many good photographers out there I wouldn't have known about if it wasn't for Instagram.