What is the best starting theme? There are lots of themes listed as starting points for theme development. A common is Do you use any of these themes, start from scratch, or have a different solution, and why?

Start by browsing d-theme.com to see themes in action and which ones validate. Look for themes that are accessible. WCAG level AA is a good minimum target.

You can create a theme as a subtheme of any other theme. Andreas1024px was the closest match for several projects so I just downloaded Andreas1024px and added a subtheme with trivial CSS changes. cuttheglow.com is a proof of concept site using a subtheme of Andreas1024px with minor CSS changes. mulders.me uses Andreas1024px with minor CSS changes and trivial changes to page.tpl.php for vertical centering. Starting from something that is close to your final requirements gives you the minimum changes to maintain.

Scratch is pure css. The whole download is just 14KB.

Skyliner is also pure CSS and is developed further than scratch. If you know CSS and do not have to support old browsers, look at the pure CSS themes.

Go look at CDMUG. Look at it several times across the day, evening, and night. Brilliant.

Zen is the most popular starting theme and has just one XHTML error to fix before reaching a good validation level. Zen Classic is a demonstration theme supplied with Zen to show you how your finished theme could look.

The main problem with a lot of big starter kits is that they first overwrite everything Drupal provides then they override all the browser defaults (usually called a reset) then they add a lot of stuff of their own, all before you add your stuff. Microsoft's Internet Explorer will load a maximum of 32 CSS files then stop. You can easily reach that limit. Drupal produces a lot of good output needing only some nice CSS. Conserve your time, your server resources, and browser resources by using themes that use the Drupal output and Drupal CSS where appropriate.

When you do need something radically different, pick something close to what you want off the shelf. I see people standardizing on a big starter kit then overriding almost everything in the kit. The result is a huge theme added onto a huge theme. Pixture Reloaded is an example of a theme that was based on Zen but all the duplication was removed and files merged to reduce the overhead. If you can find a closer match in the first place, your customisation workload is reduced and the final theme remains light.