That depends on the goal of my site.
Adsense
If the goal is Adsense CTR, I have good results with the simple looking "elements of seo" Theme.
Product Sales
If it is for products sales, I use portfolio themes with big pictures in the center of focus.
Bigger Info-Site
If it is a portal or bigger, then I got good results with The Station from Woothemes, because of its static frontpage and cms characteristics. You can optimize for a sales process on the frontpage. This theme costs some bucks, but it was worth it.
Edit: Use robust themes without complicated features. Test it in FF and IE, with and without Javascript. Often, less is more. Do not be afraid of empty space on your site. Often, simple themes without tag-clouds, flashy javascript effects, flickr-plugins and twitter streams perform better. User come to your site for a reasen and the want sites that simply do work.