For me, it was perhaps simple-scan, a very simple and efficient GUI to scan documents. I used it with my Brother printer / scanner and it works like a charm. Especially since I do not scan stuff often, so a program with more complex UI would have the effect that I forget how to use it until the next time.
KDEConnect, probably the best (only?) to do what it does in such a magnificent way
The entire KDE Community is incredible. From KDEConnect to Kdenlive, Krita, and Plasma I am a very happy nerd.