You need to be sure the SSL provider (or CA) is globally trusted in all browsers, meaning you don't get those popups telling your website visitors that the certificate has been issued by an untrusted signer.
Then you have price, support, site seal etc. So it depends on how you compare those things and what matters to you. Some certs are higher priced but come with excellent installation support and also name brand recognition.
There is no "Best" certificate per se, but there are differing types ans validation levels for ssl certs. You have Organization Validated (OV), Domain Validated (DV), Extended Validation (EV), wildcard, ucc (or Exchange), and single domain certificate.