Lubricants

Water, silicone or hybrid. Your toy's material makes this decision for you, and getting it wrong ruins the toy rather than the bottle.

This is the cheapest thing in the category and the one people get wrong most often. The choice is decided by what your toy is made of, not by preference, and the consequence of getting it wrong lands on the toy rather than the bottle. Everything else — glycerine content, pH, whether it comes in a pump — matters, but it matters second.