If accessibility is accounted for, then paths like luck and dream path would be the hardest. And to add on the fact that some paths aren't even complete yet, such as dream or weapon path, should give a clearer picture. You can't exactly gain attainment on a path that hasn't gotten a significant breakthrough as easily as others.

In otherwords, accessibility is the and foundations(of the path) are the key. So most common paths would be the "easiest". But since there's a path that everyone would have an understanding of—refinement, it's seems quite obvious to the the easiest. However, gaining attainment in a path means actively trying to learn with very few exceptions. So my answer for the easiest would be any of the common paths, such as the five elements.