well a very good question it all depends on the app that u are developing for is it a stand alone app or is mobile web app.But if it is a mobile web app i recommend html 5 ,why u might ask ,becouse html 5 has rich extensive library for building a mobile web app.which include canvas drawing ,GeoLocation API ,video and audio streaming support,offline web storage etc.like i said it all depends on ur app and wat it is going to be doing.I also recommend u to check out these html 5 mobile web development tools DHTMLX TOUCH,GOOGLE WEB TOOLKIT