For example: quality (not generally, but versus your actual input data), terms & conditions of what you can do with the data, support, data enhancements (things like timezones, etc, etc), ease of use, documentation, terms of payment, and more.
The only real answer to "which geocoding service is best" is "it depends".
We have a comprehensive geocoding buyer's guide on our site: https://opencagedata.com/guides/how-to-compare-and-test-geoc...
Please get in touch if you need geocoding, hapyp to tell you if your needs are a good match for our service. Happy also to tell you if not.