If you're looking at it from an accessibility point of view, the answer is "it depends..."
Unfortunately choosing a colour combination that is most accessible to everyone is impossible, because different people have different requirements. Some people are best helped by very high levels of contrast, whereas that can cause problems for others.
As a general rule, I would say that very pale colours are better than dark colours. Black on white is not ideal and can cause difficulties for people with dyslexia among other things. Black on pale grey (eg #eee) is better, or very dark grey (eg #222) on white. A pale green background is best of all for dyslexia ... but you've also got to think how it will fit with your site branding, and sometimes certain colours aren't going to work for you.
There are some good articles at [Lighthouse International, [url=http://www.peachpit.com/guides/content.aspx?g=webdesign&seqNum=277]Peachpit, [url=http://www.rnib.org.uk/professionals/webaccessibility/designbuild/colour/Pages/colour_contrast.aspx]RNIB and [url=http://accessibility.psu.edu/colorblindnessdetails]Penn State](http://www.lighthouse.org/accessibility/design/accessible-print-design/effective-color-contrast).