If you really want the best, these guys are the way to go. I use this Formidable for multiple sites now and far exceeds my expectations. I have used a lot of free plugins and would have never thought to purchase a form until I ran across this one. I needed it for advanced usage and wasted a lot of time w/ the free plugins. You can also use this form w/ an add-on to customize your registration form. They offer other add-ons too that seem to be useful to a lot of other people such as the MailChimp and PayPal.
The support staff actually know what they are doing and are great at explaining all issues to a novice like me.