WhatsApp. The desktop version has very few features, requires constant connection to a mobile phone and gets out of sync very often. It's practically irremediable if you're in a crowded wi-fi area and ethernet is the only way to get a good connection. It's also designed so no conversation is ever private despite advertising it's E2E encryption. Everyone you talk to has automatic backups enabled and they're stored unencrypted in Google Drive. And the "two step verification" password is the one of the dumbest things I've ever seen. It must be a 6-digit number that requires you to type it constantly in order to remember it. It basically assumes people are too incompetent to use password managers or simply writing a password down. Passwords you can remember are never safe.