Created by John Albin, Zen is perhaps the most widely used Drupal theme. Zen is a mobile-first responsive base theme with Sass/Compass support. Hundreds of thousands of Zen installs are reported by now. Zen benefits from a big support community which provides answers to questions, documentation and bug fixing solutions.
Zen is very popular because it has been around since 2006. This makes it one of the oldest Drupal themes today. It is also a very accessible theme which has RTL support. It balances tried-and-true, rock-solid solutions with the latest front-end development.
Zen ‘received’ Sass and Compass back in 2011, before most Drupal developers were thinking about it. On the other hand, it did not pick up all the fancy plug-ins that might not be widely used or officially supported.
Zen is extremely non-opinionated (about how code is structured, how layouts are built, or really anything that base themes are often opinionated about) and quite lightweight.
There are a few cons of using Zen too. For instance, finding up to date documentation can be rather tough, since Zen is quite an old theme. It is also not one of the most productive themes if you’re comfortable with opinionated themes and convention over configuration (a common feature of responsive design). However Zen can be successfully used to create responsive design apps that look and feel very cool to the end user.