Assume that the app will include a back end API and a back end data store, but that they need not necessarily be constructed with the same technology as the front end (the app on the mobile devices). By "technology", I mean programming language and/or framework and/or platform.
Gosh, there is no one best answer. The answer depends on what you're building. That said, I'm a great believer in rapid development and getting a beta in front of customers fast so you can begin the learning curve journey validated by experience rather than one's projections or suppositions. The fastest prototyping I know of is a combination of Firebase.com for the backend and a more general approach on the front-end to cover more mobile platforms, so HTML5 in combo with Ruby on Rails. Again this is for rapid pilot to fully understand your needs. Once you're sure of platform, I'm still a bit more biased toward native mobile development focusing on delivering an awesome app on iPhone, then porting to Android and others.
After drafting the above answer, I was reading the latest Gartner trends report and found information relevant to the question:
Gartner Identifies the Top 10 Strategic Technology Trends for 2013
Analysts Examine Top Industry Trends at Gartner Symposium/ITxpo, October 21-25 in Orlando
Mobile Applications and HTML5
The market for tools to create consumer and enterprise facing apps is complex with well over 100 potential tools vendors. Currently, Gartner separates mobile development tools into several categories. For the next few years, no single tool will be optimal for all types of mobile application so expect to employ several. Six mobile architectures – native, special, hybrid, HTML 5, Message and No Client will remain popular. However, there will be a long term shift away from native apps to Web apps as HTML5 becomes more capable. Nevertheless, native apps won't disappear, and will always offer the best user experiences and most sophisticated features. Developers will also need to develop new design skills to deliver touch-optimized mobile applications that operate across a range of devices in a coordinated fashion.