Our worst nightmare is when we hear “I’ve got a custom WordPress theme built from scratch.” A lot depends on how well your developer really knows WordPress- and how well they document your theme. With so many advanced themes and frameworks available- where you can easily make a child theme- and let a theme developer deal with all the changes in WordPress- going the custom theme route seems kind of silly.

But- if you are in a groundbreaking industry, and have a specialized use, and a large support staff, you may want to go the custom theme route. The New York Times can do this, the Podunk Picayune probably can not.

Using starter theme in your development is a great way to create a WordPress theme with all the up-to-date WordPress best practices.
Underscores: A starter theme maintained by WordPress parent company Automattic.
Sage: A starter theme based on HTML5 Boilerplate, gulp, Bower, and Bootstrap.
Bones: An HTML5, mobile-first starter theme for rapid WordPress development.
Quark: A simple starter theme built on HTML5 and CSS3.
JointsWP: A starter theme built with Foundation 5.
Naked WordPress: For designers who don’t know WordPress.
HTML5 Blank WordPress Theme: A WordPress HTML5 boilerplate starter theme.

Source: 15 Free Resources To Help You Become a Smarter WordPress Developer

Not a complete list- but, a good place to start