Whats the best module or method for A/B testing with Drupal?

Google website optimizer is free and used by a lot of people I've not tested it with Drupal but here's a link with step by step instructions so I would think you would have no problems http://www.drupalseobook.com/news/google-website-optimizer-with-drupal. (since it's free it's easy enough to test it first). A paid option which I use and highly recommend is Visual Website Optimizer. The cost is minimal and it should work on anything.