It depends on the persons gender, your relationship to the person you're greeting, how traditional they are, and how old they are. It's sort of tricky to understand if you haven't been around Greeks for a very long time. The safest bet is to shake hands or to shake hands and kiss on the cheek simultaneously
I wonder how much of modern Greek culture is based on the Byzantine and Ottoman empire and how much is from ancient Greece. I'm not Greek but my husband is and I'm trying to learn more about the culture, any help and opinions from all of you have been super helpfuL!