Nearly all video games, and by extension where i get them - Steam. Open source gaming is simply not an option. One can - and i do - stay away from the big annoying AAA companies most of which of just subsidiaries of yet bigger annoyinger tech companies, and Valve is actually pretty decent about open source, but one cannot simply game without proprietary software.
Discord and WhatsApp. Literally everyone i know uses one of them.
For me at least i have no problems with the UI, functionality, etc, of these apps' foss equivalents but the main problem is the userbase. You cannot have a messaging app with no people to talk with and you cannot and a game store with no games to play. You can have the best design and software ever but that won't do shit without the established legacy and publicity of a big company.
Youtube-dl to download things in flac format from youtube (and sometimes rarely other sites when what i want isn't on youtube, which is rare because people upload everything under the sun). I haven't the need for anything more complicated.