You may be surprised to discover that I use a relatively cheap ($700) Sigma lens for my 1Ds3. But the Sigma 12-24mm f/4.5-5.6 EX DG HSM is a truly unique lens: it is currently the widest rectilinear wide-angle for fullframe, at 12mm, it gives an amazing angle of view of 122 degrees! By comparison, the widest wideangle zoom of Canon, the 16-35 f/2.8 II, has "just" an angle of view of 107 degrees - an huge difference, that has led me to choose the Sigma.
 
Lens Specifications