That depends. Do you want a shallow depth of field, or a large depth of field? In other words, do you want a blurry background, or do you want everything to be sharp in the foreground and background? If you want a large depth of field (everything sharp), use full manual or aperture priority mode, and use either f/11 or f/16. Shallow DOF (blurry background) is a bit trickier, particularly in full sunlight. First of all, you need a lens that is capable of shallow DOF, which means its largest aperture has to be greater than f3.5 or f4. In other words, your lens needs to be capable of operation at f2.8, f2, f1.8, and/or f1.4. Anywhere in that range will give you shallow depth of field. The lower the number, the shallower the depth of field. In order to use these apertures in bright sunlight, you need to have a camera that is capable of some seriously fast shutter speeds. For instance, in full midday summer sun with no haze, at ISO 100, the correct shutter speed for an aperture of f2.8 is 1/4000th of a second.