If you need a free SSL certificate, then this is the Let’s Encrypt one. It doesn't require a lot of time to obtain an SSL certificate, and the process itself is simplified as much as possible.
The Let’s Encrypt SSL certificate has limited validity period to 90 days. After that it must be re-issued again. Currently, more than 70 million active domains use a free certificate, increasing the overall percentage of Internet security to record heights. Let’s Encrypt allows you to transfer your website to secure version of the HTTP/2 protocol, change the status in the address bar from “Unreliable” to “Protected” and get a lock.
Of course, Let’s Encrypt makes no financial guarantees in case of information loss and requires SSL to be re-issued every 90 days. But if your resource is not associated with financial transactions or you're looking for a certificate only to switch to HTTPS, then Let's Encrypt is a great option.