For those of you who use CloudFlare CDN in front of your website, below a couple of Page Rules to improve caching.
While reading the CloudFlare documentation, I realized that by default, CloudFlare does not cache URL that do not end with an extension, which reduces its impact, in particular for lots of WordPress sites where URLS usually end with a slash like: https://mydomain.com/categ/page1/
1. Section Caching / Configuration
Ignore Query String option. Most of you should know this already. But it will only cache images, JS and CSS files.
(By the way, it is good to know that CloudFlare Free plan does not cache MP4 files. Have this in mind if you need to serve videos.)

2. Section Rules / Page Rules
Add these two rules in the Page Rules section to cache all pages ending with a /

Be aware that it is aggressive caching, and it will work only if you serve static content. If you need to use ecommerce plugins, or if you use first-party stats plugins that store data in WordPress, they won't work. It can also mess up with the admin section with some plugins. In that case, you can temporarily disable in the CloudFlare domain homepage, using dev mode that disables cache.

This is the result on an WordPress website I took over recently, which was a D grade initially.

Nice, thanks for sharing this
From a D to an A! This is extremely impressive, not to mention helpful - exactly what I'm needing to know at this time as I'm in the process of optimizing a website right now.
You must be psychic @jeremie! 
Thank you!
Amy