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.
Sharing files and clipboard is a nice feature. Media control might be pretty neat in some scenarios (party jukebox, htpc).
What completely blows my mind every time is when KDE Connect time pauses my music on my PC when I get a call on my mobile.