Pihole is a big one for me. I also use a firewall on my phone to prevent apps from accessing the network unless I explicitly allow them.

Standard Notes is a big one for me. Thunderbird is the only way I access email on my PC. LibreOffice (or OnlyOffice) is regularly used on my end too. VLC is amazing and the only method I listen to music or watch videos on my devices. JabRef is a phenomenal bibliography tool I use for papers. Plus, Kdenlive is an amazing video + audio editing tool I use to clean up OBS recordings. Lastly, Syncthing is how I manage files across multiple devices. All of these are fantastic and a big aspect of my day to day.

For sharing files with a friend: magic-wormhole

If I had to choose the top one, it would be self-hosted NextCloud, with a focus on its PhoneTrack plugin and corresponding Android app. Finally a FOSS privacy-friendly way to share my location with my family.