When we are running with a self signed certificate on server and using Cloudflare SSL on the DNS End ,  Use Full Strict Certificate . Here is the description from official Cloudflare website :

Full (strict)

Full(strict) ensures a secure connection between both the visitor and your Cloudflare domain and between Cloudflare and your origin web server. Configure your origin web server to allow HTTPS connections on port 443 and present either a Cloudflare Origin CA certificate or a valid certificate purchased from a Certificate Authority. This certificate must be signed by a Certificate Authority that is trusted by Cloudflare, have a future expiration date, and cover the requested domain name (hostname).