What can be saftly deleted in data/data. I have almost no space left on a few devices. I don't want to delete account info.
Remove apps via Settings > Apps > Your app > Uninstall. It is safer and you will be able to know easily the app(s) that you want to uninstall (you won't need to know the package name).
Start by removing apps you don't use. If there are system apps (i.e Google play music, duo, Hangouts, etc), you can remove their updates then disable them.
If there are apps you want to remove and keep their data in case you reinstall later, use Titanium backup or Helium to backup the apps and their data then remove them.