The classic desert island question: Which lens?

It's easy for me to pick a focal length.  I choose 100mm or the 35mm equivalent.  On my old Hasselblad that meant the 180mm f4.  It's a lens I still miss even though I sold my copy nearly ten years ago.  On an Olympus 4:3rd's camera, like the e30, it's resolutely the 50mm Macro.  Or the equivalent focal length on my 35-100mm zoom.  I the Nikon system it's a toss up between the 105mm f2 DC lens and the old but incredible 105 2.5.  The lens I like on the Canon is the standard 100 f2.  But all of those choices are easy ones for me because I know I love the focal length.  I can't seem to get away from my need to cut away background and focus in on my subjects.  The real question is,  if I could only choose one (and my accountant tells me that after last year's business performance I might want to start making choices....) which one would I rescue from the sinking ship?  The easy answer is the Hasselblad 180.  But reality tells me that film is fading away from the market and, additionally, once the old V bodies are gone there will never be another camera system to use the same mount.  I'd like to cheat and choose a lens that will be usable with future cameras, if possible.

That narrows it down a bit.  I love the Olympus 50mm f2 and I think it may be the sharpest of the other lenses but......it's slow to focus and I'm waiting to see the future roadmap.  Will they make a version that focuses like lightning on a m4:3rd body?  That's what I'd love to see.  The lustre still hasn't worn off of my little Olympus Pens and I'm still totally fascinated by how attracted I am to the previewable nature of the EVF's.  That leaves me with the two big boys:  Canon and Nikon.  And I have to admit it would be a toss up.  If I chose Nikon I would go with the 105 2.5 because it doesn't have the weaselly "G" configuration and that means I could use it on just about any camera body out there with the right adapter.  I could stick it on a Nikon D3x for those clients who need relentless megapixels and I could stick it on the Pen EPL when I just want to have fun.

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.

Anybody game?