I am working with a client who hired a designer to custom design a landing page. I suggested starting with a template and then replacing individual sections piece by piece.

Then I started thinking maybe something like Webflow is actually best but I'm more focused on the backend so I'm not sure what's best in 2018.

Webflow? Wordpress? Squarespace? It's basically a landing page that should eventually be hooked up to a CMS system and get a custom design.