WINNER: 'Hi [name], ... '
If you want to make it a little more formal, you can always use the person's last name: "Hi Mrs. Smith, ... "
"The reason I like this one is that it's perfectly friendly and innocuous," says Schwalbe.
It's also Pachter's favorite. She says it's a safe and familiar way to address someone, whether you know them or not.
So when in doubt, go with "Hi."