To facilitate the development of a theme, sometimes we need a starter theme for accelerating the development process, especially if we have a limited time. With a starter theme, the development will be much easier because of existing frameworks.
Zen starter theme is a very widely used, and has great online documentation so it is very helpful if we want to study it more deeply. It very flexible, standards-compliant and semantically correct XHTML theme.
Zen is very simple and clean, and the HTML source order has content placed before sidebars or the navbar for increased accessibility and SEO. Zen also has a lot to get the award, which is clear evidence of efficacy from this starter theme.
Framework is developed by Andre Griffin, very user friendly starting point to help facilitate theme development.
Framework has general placement and formatting to basic Drupal elements, and supports one, two, and three-column layouts. The CSS file is highly organized, very clean and simplified code.
Genesis is easy to theme and extremely flexible, giving themer endless theming possibilities. Genesis makes building a standards compliant accessible theme lot easier than starting from scratch.
Genesis has no complex logic in templates, included conditional styles module, and the content source ordered for SEO.