You will find a lot of different answers to that question depending on who you talk to. I have used the thesis theme a few times as many people say how good that is for seo but honestly I found that any quality built theme is just as good. It is more on using the right plugins and knowing on page seo. Although thesis did make it easy to optimize each post (for seo). So it might be worth you taking a look at the wordpress thesis theme, just Google it.
don't look for SEO themes. google is actually starting to look for themes that are "seo themes" to use as spam footprints. Take any theme and tweak it for SEO. if you don't know SEO. learn it.