Learning and working on a Drupal starter theme can not only speed up workflows significantly for 'themers', but also enable you to focus solely on building custom layouts without having to worry about browser incompatibilities and setup. Starter themes provide a strong foundation for front end developers to build on. Below is a list of not only popular but the best Drupal starter themes commonly used by 'frontenders'.
Zen Theme - Zen Theme is a veteran theme in the Drupal Community, having won the title of “Best Drupal Theme”. This theme is highly recommended for all levels of Drupal themers, as it is packed with Drupal and Front End best practices. It is also loaded with a plethora of features focusing on solving numerous browser incompatibility problems. This is one of the very first themes I have tried and have learned a lot from.
Fusion Theme- Fusion Theme enables users to control the theme within the Drupal UI; it is designed to be used alongside Fusion Accelerator Module which allows users who do not have traditional theming skills to create advance layouts within the Drupal admin.
Framework Theme- Framework Theme is perfect for puristic themers. It provides all the basic themer needs on a simple and clean HTML5 markup. The filesystem is highly organized, making Framework a perfect theme for themers who don’t necessarily want to reinvent the wheel, or remove unnecessary code every time.
Basic Theme- Basic theme offers a simple, smart, and flexible theme. It comes with complete SASS support, and a variety of commonly used mixins as well as a SASS based grid system. Hence, this theme can be molded to themer's any type of workflows and layouts. This is my current go-to theme as it is amazingly flexible to meet all my requirements as a themer.
Adaptive Theme- This theme shines for content heavy websites, such as social network and forum sites, that require responsive design techniques. Adaptive Theme aims to solve cross browser responsive theming problems.