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.
Microsoft Defender.
I convinced my work to let me use linux on their laptop. They sent me instructions for setup. One of them was to install Microsoft Defender, had a link to the Ubuntu package and everything. Blew my mind.
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.
Bitwig and Reaper. Two of the best music DAWs on the market and they each have a Linux native version.
EDIT: I forgot to include Renoise, the music tracker DAW.
As a guitarist ToneLib and Carla are also up there for me.