Choosing a health insurance provider in Switzerland?

CSS is the largest healthcare provider in Switzerland and Swica is super pervasive in the Zurich canton, either would be decent choices. If you're coming from the US there's some differences that you'll have to familiarise yourself with when it comes to paying premiums.

The Swiss government creates rules for a universal basic package that all insurance providers must offer clients. This covers in-patient, out-patient, care for the elderly and disabled, hospital stays, consulatations, etc... It also covers most of a prescription drug cost (though for brand-name pills you might have to pay a small co-pay). Though the package is the same everywhere, the costs will differ according to where you live, not how much you make.

Just one of those things. You can reduce costs by choosing a slightly different model of the basic package, each insurance provider has variations and then discounts attached.

For more info you should check out the Expat Arrivals Healthcare in Switzerland page. It has info on average costs and specifics.