I used to believe that the two most important refactorings were Extract Method and Rename. The way they deliver value and the way they are used are quite different, so it's hard to compare, so I figured they had equal value.
Recently I've decided that Rename is slightly more urgent, if not more important. It is the first refactoring to learn; the first to teach; the first to apply. (Just slightly)
The problem is code that lies to you. It says it's doing one thing, but actually it's doing another. You either have to think really hard to figure that out (slow) or you misunderstand the code and write bugs.
Fix that first. It may lack cohesion, have tight coupling, and lots of duplication, but first introduce good names. Rename to make the code stop lying to you.
(Soon afterwards, start using Extract Method to give you more things to name.)