I believe MXM has done testing but my guess would be Sandisk Class 10 but "best" can change very quickly in any direction. Would you believe I'm using Sandisk 32GB Class 2 cards purchased in January 2009 and they work fine. I think there was a small run that far exceeded their class rating and that shortly thereafter even the Class 4 cards had issues.
See this by MXM Express
http://mxmexpress.com/cms.php?id_cms=6