Most postapocalyptic worlds I've seen simulated so far had at least some kind of monastery to hang out in, even if its other inhabitants are often less than helpful and usually need to be blown into a fine red mist. So it probably comes down to CPAN and toilet paper.
Yesyesyes, clean water, that's a close third of course. Living in a place where tap water has over 10 times the concentration of harmful bacteria than the WHO-recommended maximum for public swimming pools plus a *cough*healthy pinch of salt I know a thing or two about clean water.
