Chess improvement can be overwhelming, there's so many things to learn. Depending on your level you'll need to study different subjects.
Still there's one that it's a must for players all levels: Tactical training. You need to keep your tactics, sharp as a knife. Need some evidence?
World Champion Tigran Petrosian said it "In chess everything depends on tactics".
GMs Axel Smith and Hans Tikannen also agree on training tactics, check their books: PUMP UP YOUR RATING and THE WOODPECKER METHOD.
Worldwide known chess coaches like Dan Heisman also stresses "Tactics is undoubtely the single most productive area that beginners and intermediates can study to improve their game -the more practice, the better".
And amateurs themselves, also agree. E.g. Michael de La Maza in his book RAPID CHESS IMPROVEMENT explains how his rating increased using tactical training.