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.
I really like LocalSend as well, but it’s very inconsistent with me. I think it has to do with one device being on a VPN, but I’m not totally sure. Basically I have some “one way” connections where one device can see and send to the one connected to a VPN but not the other way around. Is there some way I can specify LocalSend connections to ignore the VPN? I’m on NixOS and installed LocalSend in my user package declarations in my Nix config.