Great thread with lots of interesting examples.
If I could only keep one:
Very tough choice, as I have nearly a dozen Omegas and I'm having a hard time letting go of any of them buy given the question... If we're talking for daily contemporary living, sitting at my desk, being in meetings, etc., I'd probably choose my AT Worldtimer for its versatility, looks and practicality.
If we're talking only one to stick with you through the end of days, it would have to be my PO 8900. Great looks, tough as nails, could follow me to as far down the depths of the sea I could ever possibly reach and can probably survive an electromagnetic war. Great back hand weapon too, all while wearing very comfortably.
If we're talking which one watch in the whole world you must have, I haven't found it yet. It would be something like an SMP with the dial of the latest PO, minus the He valve. It would also work in the case of a 300 Heritage. Then I'd have to have it in both blue and black.
I've yet to own a Speedmaster, as I'm a soul of the sea, but I'm thinking about it. Maybe it'll help me see what all the allure is about.