For me it’s Whatsapp. Like 99% of people i know use it so yeh… i still need it. I am considering moving the Whatsapp Gdrive backup to a local backup btw.
Google photos is also a major problem for us. We have a lot of stuff that want to migrate to different service (or store locally).
I’m still working to get rid of google, I’m worried there’s no way to transfer all my photos from google photos and I still use the tasks app and calendar.
For the things I did switch, I found a suite of apps under the name Fossify which I use for my gallery, phone app and file manager.