For aircraft watching you'll need high power/magnification to see the detail.
For hand held use we wouldn't suggest anything higher than 12x, look for a 12x50 or 12x60, if you are carrying a lot of equipment and size is important to you, a 12x25 or 12x30 will work well in daylight.
Used with a tripod, monopod or other support, higher magnifications would be useful. Many aircraft spotters use a 20x50 monocular to achieve high power in a compact body, a 20x60 binocular is another good choice. For a brighter image with high magnification consider a 15x70 or 20x80 model.
Anti-UV lenses can be helpful, these increase sharpness in bright, hazy conditions.