well, just to stir the pot ...

... the "tool" I would choose (if forced to choose only one) would be a slightly oversize gen-2 goretex parka, knee-length with full hood and drawstrings everywhere ...

... it's practically a shelter in itself when you batten everything down ... it adds to the effectiveness of any clothing you have ... you can add insulation by stuffing in any semi-dry source you come across (grass, reeds, etc.)

... thinking back to my years of solo mountain backpacking, this would be the most critical item for me, hands down

(btw: this is a quite the admission from a total gear nut, knife/axe/multitool accumulator and flashaholic)