In terms of hard-line necessity, either Agua Lavanda or one of the Guerlain eaux. Which doesn't much matter, as their merits and disadvantages seem to equal out. I would no sooner try to get by without one of these than I would without soap and water.
However, in the realm of actual fragrances (the aforementioned are more hygiene products), Monsieur de Givenchy is foremost among the fragrances I hope to always have to hand.