Easiest is pretty difficult to define. It depends a lot on what I have already deployed in your system, your knowledge and your necessity.
A external HD disk is the easiest for me. It is only a local drive without any authentication requirements. It is the perfect solution for a quick and easy restore. But it lacks the reliability of a offsite solution, that is based in a more fail proof architecture. It is certainly a very useful piece of a complete backup solution, and for it perhaps you can even skip encryption.
But if you have already a virtual folder configured in your computer, like a dropbox, it is almost the equivalent.
Other solutions are also pretty straight forward assuming you have already signed for the service, and know your user and password.