Although I don't build any apps, I've been using Lato a lot lately (my own website/resume, company's site) and it works really well. I didn't want to go for the well known ones ;)

I generally use: Calibrie, Verdana, Neuropol, Arial and/or Sego UI. Otherwise Open Sans or similar.

I have mostly used "Open Sans" and "Roboto" in most of my projects.

• For corporate companies: Open Sans works.
• For freelance projects with Material Design : Roboto!