What is the most secure way to submt a form

Well if you are taking about username and password forms generally the security and code to make the login happen will generally be handled by a backend language such as Ruby or PHP and only the form itself will be handled in html. If you mean something different please correct me.

This is my personal opinion and is in no way researched fact:

There is no single secure way to submit a form. My first advice is use POST instead GET since you are not including the password in the URL. If you want to further improve security of the transfer of data, it all depends on what type of attack you are expecting. For instance, to combat man-in-the-middle hacks, you would encrypt the form values from the client side using javascript before sending. Of course, the encryption method would've been caught by the MITM anyway, it simply makes it more difficult and thus a less desirable target. This has the drawback that the client browser has to support and enable javascript, and on the server-side you need to be able to efficiently decrypt the data, which can be time consuming depending on the method.