Best thing, definitely “Learning how to Learn”;

• Since school days, my pursuit has been identifying what makes certain people more successful than others. The connecting link between every successful person is their ability to learn, adapt and grow quickly.
• Knowledge by itself is of no use if you cannot apply it in the correct way and make something useful out of it. Simply accumulating facts and figures without being able to put any of it to use is meaningless. This is why the Indian Education system fails miserably at creating competent workers.
• In one of MIT's Introduction to AI course lectures on YouTube, Patrick Winston says “Knowledge about Knowledge is power”. When you learn how to learn, you force yourself deeper into the abstractions you're trying to make sense of. You learn to think (somewhat) using a First Principles approach.
• I learnt more about human cognition and how the brain processes information. Having these things at the back of my mind makes me appreciate why I use certain productivity hacks instead of blindly following tips all over the Internet.
• Learning how to learn, eliminated all the mindblocks I had about disciplines that were not related to my area of specialisation. That was sheerly a matter of self-belief because it overcame my learning limitations.
• Elon Musk built SpaceX after reading books on rockets for a year or more. There are many people who read those books, only Elon Musk built SpaceX, why? I'd say it's because of his ability to quickly grasp and connect ideas from multiple disciplines and that comes by being an effective and fast learner.

How I reached these conclusions?

• I read an answer by Aman Goel where he talks about “Learning to Learn” , couldn't find the link to it but it's a pretty great answer.
• There's also a Coursera course on the same by Barbara Oakley which talks about issues like procrastination, time management, breaking concepts down and other learning techniques that were immensely beneficial to me. I then moved on to her book (A Mind for Numbers) for a more detailed scientific understanding. Learning How to Learn: Powerful mental tools to help you master tough subjects | Coursera
• To sum it up, read this article by James Clear which talks comprehensively about Mental Models. How to Train Your Brain to Think in New Ways

I'd recommend all my readers to spend time with these resources to upgrade your mental CPU. ;)