A good lens attachment enhances the zoom and wide angle capabilities of your iPhone, most models of which come with fixed wide angle equivalent to 29 mm at most. 

The best telephoto lens for iPhone photography: Moment Tele (easy to use, 2x zoom on your camera, can work with slim phone cases)

The best wide-angle lens: Moment Wide (get more detail in your picture)

The best lens for quality images and more features: iPro Lens System

The best affordable lens for iPhone pics: CamKix Camera Lens Kit for iPhone 6/6s ( fisheye lens, combo macro, wide angle lens, 8x tele which requires the included mini tripod.)