I agree with a lot of people on the poll that Early Edition is one of the best themes to have as a default, as it's accessible in a lot of way. However, it customises the navigation strip such that the non-link text is a dark red, which makes it hard to see against the black background. Also, many people have a problem with a pure white background.

My preferred theme is Broad Horizons (preview using your journal) because it has clearly coloured links, clearly delineates different parts with lines, and doesn't mess with the navstrip. The pale green background isn't so great IMO, but its probably better than pure white.