Absolute best way to improve

I don't think there is an absolute best way. If it was me, I would buy a few good books on chess with annotated games in them. And I would read the book, with a board next to me, and play through the games, reading the annotations and trying to understand what was played and why. But that is the best way for me not for you. We now know that there are different types of learners (in education generally not just in chess) and different learner types need different learning methods. You need to find what works well for you. 

I don't know.

But I'd guess something like a tactical puzzle book with puzzles grouped by theme (pin, fork, removing defender, back rank mate, etc.) and an annotated game collection book.

Something like these for example:

https://www.amazon.com/1001-Winning-Chess-Sacrifices-Combinations/dp/0879801115

https://www.amazon.com/Masters-Chessboard-Richard-Reti/dp/092389148X

And of course playing games, and reviewing all the games you play (with help of engine and database).

I was in your position 3 years ago. Play many games online (20min is good) and OTB, watch a few YouTube videos (I can suggest a few channels) and practice tactics like there is no tomorrow (download an app on your phone). That was my method to improve, it may work for you. Take into account that I did not invest a single penny in chess, that is the reason why I don't mention books, I haven't read any, people say they help but I have no first hand experience with that. If you have any questions feel free to send me a message.

Possibly helpful:
Simple Attacking Plans by Fred Wilson (2012)
https://web.archive.org/web/20140708090402/http://www.chesscafe.com/text/review874.pdf
https://www.newinchess.com/Shop/Images/Pdfs/7192.pdf
Logical Chess: Move by Move by Irving Chernev (1957)
https://web.archive.org/web/20140708104437/http://www.chesscafe.com/text/logichess.pdf
The Most Instructive Games of Chess Ever Played by Irving Chernev (1965)
https://chessbookreviews.wordpress.com/tag/most-instructive-games-of-chess-ever-played/
Winning Chess by Irving Chernev and Fred Reinfeld (1949)
https://web.archive.org/web/20140708093415/http://www.chesscafe.com/text/review919.pdf
Discovering Chess Openings by GM John Emms (2006)
https://web.archive.org/web/20140627114655/http://www.chesscafe.com/text/hansen91.pdf
Openings for Amateurs by Pete Tamburro (2014)
http://kenilworthian.blogspot.com/2014/05/review-of-pete-tamburros-openings-for.html
Chess Endgames for Kids by Karsten Müller (2015)
https://chessbookreviews.wordpress.com/tag/chess-endgames-for-kids/
http://www.gambitbooks.com/pdfs/Chess_Endgames_for_Kids.pdf
A Guide to Chess Improvement by Dan Heisman (2010)
https://web.archive.org/web/20140708105628/http://www.chesscafe.com/text/review781.pdf