[arstin]

Atomic is has been my choice for a long time now and they keep making it better. It's best used as the UI flow tool, complemented with something like Pixelmator and Sketch for asset creation and visual detail work.

If I have to show other people very early stage ideas I still use Balsamiq for the artificial lo-fi effect, but with other designers who have more control over their imaginations I'll just throw something together in Atomic.