If you own silicone toys: use water based, or read the bottle every single time. This is the most expensive mistake in the category. Silicone lube on a silicone toy is not reversible, and a good dildo costs more than a lifetime of water based lube.
If you're in the shower or the tub: silicone. Water based lube washes away the moment it meets running water. Silicone lube stays put, which is most of the reason it exists.
If you're prone to yeast infections or irritation: read the panel for glycerin and sugar. Both feed yeast. Glycerin-free water based formulas exist for exactly this reason and we stock them.
If it keeps drying out on you: add water, not more lube. Water based formulas absorb rather than evaporate, so a few drops of water brings back what is already there.
If you're using condoms: skip anything oil based. Oil degrades latex. Water based and silicone are both condom safe.
If you want one bottle that covers most of your life: a hybrid. It is the compromise formula, longer lasting than water based and easier to clean up than silicone. Start with the hybrids.