Zeiss Planar T* 100mm f/2.0 M/M Lens

The down sides are- cost and bulk. The upsides are- extremely bright through the viewfinder aids focussing, even in low light; very sharp even at f2.0; weight actually helps in balancing with a Contax camera; 100mm focal distance lets you catch portraits with the subject unaware of what you are doing for a more natural pose (which also works for wildlife); at f2/2.8/4 you can throw the background out of focus for startling in-focus images; highest build quality.

With an adapter, it can be used on Canon EOS film and digital SLR's. On digital SLR's the focal length becomes 160mm, but the aperture stays at f2. A 160/f2.0 telephoto lens. Just imagine. Of course, you focus manually, you must sop down manually and you can't use auto metering.