IMO, Python. However, the reward isn't immediate. What I mean by that is you won't be able to make something tangible that you can see or get excited about, initially. One thing that Python will do is teach you is how to translate what you have in mind into a code without much syntactical voodoo.
Also, if you really want to put some effort and you understand high school mathematics well enough, I would suggest a read of Structured Interpretation of Computer Programs by Sussman & Ableson. It would connect the dots between Mathematics & Computer Programming and teach you some advance concepts which all JS developers swear by. Although, keep in mind it is almost all Scheme (a very old programming language, but excellent for learning).