From my experience, those that alter the contents during lessons for sure. The biggest one being the Keisei-script. That one is pure gold and makes a huge difference in how you learn kanji readings and understand relationships between them from radicals.
Others that also adds things to how you learn are:
Those are probably the most important ones for improving upon on your learning experience overall during lessons and on info pages. Also works during reviews as well, when you check info.
Outside of lessons, I’d say “Self-Study Quiz” and “the Item-Inspector” are both hugely helpful with your learning process. And of course things like the “Heatmap” and Ultimate Time-line are as well. But, concentrating on what adds “content improvement”, the above linked scripts wins out for me.