After researching and testing many DotNetNuke Skins and if you have a low budget I recommend the following free skin
Greytness by Adammer - http://skinsbyadammer.com/Default.aspx.

When designing a custom skin I would use Greytness as a starting template to start from. The skin code and layout is well written and easy to understand.

This skin is W3C compliant supports DotNetNuke 4 & 5 and speeds up performance.

The download location is http://skinsbyadammer.com/LinkClick.aspx?fileticket=F08s-jLusV8%3d&tabid=81&mid=454

A demo of this skin is located at http://skinsbyadammer.com/Greytness.aspx

Adammer blog location is http://www.adammer.com/blog/2009/07/adammer-releases-greytness-a-free-dnn-skin/comment-page-1/#comment-68