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.
LocalSend.
No more USBs ever (outside of install media). So so simple, fast, and works on all devices and FOSS.
It is really the best UX of any file sharing app I have experienced (outside of airdrop I guess, but obvious problems there)
Okular is also a favorite of mine.