The activity that I do which is most likely to damage a watch is mountain biking. So reluctantly I would have to say my one watch would be my G Shock. I rarely need to wear a suit so it technically would be fine 99% of the time.
However if I could go watchless on the mountain bike (or stick it in my pack) my one watch would be my CWC RN Diver. Tough enough and waterproof enough for everything else I do and can be dressed up a bit with a nice strap.
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