The Lens
I've been getting some use out of the Rokinon 14mm f/2.8 (or the Samyang 14mm f/2.8 or Bower 14mm f/2.8, these lenses are identical except for the name) lately. It's the cheapest non-adapted lens that I currently own for the Canon EOS 5D Mark III, but really seems to hold its own against far more expensive options on the market. It's also the widest angle rectilinear lens you can buy for a full-frame Canon DSLR. Anything wider than this is a fisheye.Sure, you could also opt for the Canon 14mm f/2.8L II, which gives you autofocus and in-camera aperture control, but for those conveniences you'll be paying $2000 more than the price of the Rokinon. There are other lenses that I'd get more use out of that I'd rather buy with that nonexistent money.