any that mean something would fit into this category. (ip, eu, etc.)
Other than that, those that are easy to type would get the vote, not necessarily for specific letters (because they're all freely available on the keyboard), but variants that make them easy to find and type. For example, two of the same letters (aa.com, bb.com, xx.com, etc.) would find more users and type-ins, etc., and that would add to their value.