The classic desert island question: Which lens?
The plus for all the older, manual focus, Nikon lenses is how great they are when used as "cine" lenses. The focus ring is more linear than that of AF lenses and allows one to do a good rack focus. I can still use it on my old Nikon F, as well.
Edit: Monday April 29th. 2:03 PM. I should have been more clear. It is a desert Island (whatever that means) but it is not deserted. There are a bevy of supermodels, several chefs and a crew of attendants. We were shipwrecked and you were able to swim ashore holding one lens out of the salt water. All the necessary cameras (sans lenses) washed ashore in a Pelican Case. A cargo plane previously crashed on the Island with cases and cases of fine wines and interesting foods. Miraculously the refrigeration unit is still functioning and will continue for several years given the solar panels and storage batteries that were also on the plane.............add your own restrictions to the story as necessary.
The Canon 100 f2 is faster and the autofocus works well on most of their professional cameras. Not so great for crossing over to m4:3rds.....
I was going to write off the Leica M lenses because of the paucity of bodies for digital but now, with all the adapters to m4:3rd I'm playing with those too. While the 90 Apo Summicron may be the sharpest of all that alone makes it a bit less practical for me as a portrait lens.
Enough. If you were constrained to shoot with one lens what would it be? And what camera would you put it on?
To make this sensible let's disqualify all zoom lenses if your intention is their flexibility. If you truly believe that a particular zoom (at a particular focal length) gives you just the right look then go ahead and tell me that.