For me it is KDE Plasma desktop. By far the most powerful, customizable, flexible and innovative of the computer desktops today. And despite all these features and power it is still one of the more lightweight ones.
Also there are other great KDE apps and GNU/Linux operating system in general. Just love how they respect my digital rights and freedoms and my privacy and I do not need to use proprietary alternatives which are just getting more bloated and include more and more spyware. Not to mention their constant dumbing down of the interface and closing it up.
Firefox is the first to come to mind. Also all the KDE software (when run in KDE).
Jellyfin
Blender
Darktable
Bitwarden / Vaultwarden
Yep, there are quite a few forks of it too, eg. one for artists with a friendlier UI, a CAD-focused one, a game engine one etc.
Linux! The responsible for my knowledge in computing and a great deal of English…Linux is the power!
I switched to Linux Mint full-time a few months ago and it’s blown me away with how good it is.