You can't go wrong with paid themes from elegantthemes or from reputable themeforest sellers.
Just pick one you like (they're all responsive, it's a standard these days ).

Yep, i just recommended StudioPress to others as well.

Great themes, easy to customize, and awesome support community.

I can't agree more with what the previous warriors said but if you want some specific themes to be proposed I can give you some names from a past outstanding experience building websites with them:

1. Avada - Avada | Responsive Multi-Purpose Theme | ThemeForest
2. Enfold - Enfold - Responsive Multi-Purpose Theme | ThemeForest
3. The7 - The7

Make sure you will check these out as well as other options.

Regards,
Rares Michael

Hi, you can design WordPress themes by yourself as well using a WordPress theme generator. Using such tools you can easily design a variety of fully responsive themes in a lesser time. Themes made with such tool will surely be mobile friendly and SEO friendly.

Hope that it will help you

https://colorlib.com/wp/free-wordpress-themes/
WordPress is indisputably the most preferred CMS platform on the web. Due to the popularity and development of WordPress CMS, themes that are specially made for sites powered by WordPress also gained considerable fame in the market. Users love WordPress themes because it is painless to set up and can be used to create a website like blog, personal, corporate and portfolio site. However, quality free WordPress themes are not easy to find that is why I decided to gather some of the finest free WordPress themes on hand. Most of these themes include astonishing features that will certainly blow your mind. So, here’s the list of the best free WordPress themes that you can use for your next project: