You're getting a lot of replies that aren't mobile apps, but just software products in general.

In terms of mobile apps my money (hah!) is on one of the many medical apps that are in development - we're talking about apps intended for use by medical professionals in actual medical institutions.

Not only are they extremely costly to develop (someone's got to pay the army of consultants, after all), they face incredibly costly certification processes before they can see any actual users.