Recommended upgrade for my civiCRM with Drupal 6
You don't mention any extensions on your site so if that is not an issue then I think you are as well going to 4.7.x

If you do have extensions then you might want to check things out before going past 4.6.x which is a LTS (long term support) version and has probably had more extensions verified for it than for 4.7.x

Of course if you do have extensions, it helps the community to know if you find any issues on 4.7.x and reporting them back to the maintainers or contributing patches or funds to help ensure they do work on that release.