Most websites are built using the CMS (content management system) WordPress. I prefer to use this platform since it is cost effective and future proof for the client. As of March 2018, around 30% of all websites are using WordPress.
If can also design using Bootstrap framework if WordPress is not wanted. Projects will usually run a little higher since it will take more time to code these sites out.
All websites will be SEO friendly and fully responsive across different device resolutions.