Much like dating, first impression counts.
For the greeting, it’s best to stay professional but not stiff or too loose. Something casual like “Hey you!” is generally a bad idea, as is “To Whom It May Concern” because it sounds boring and automated.
The best approach? Something that doesn’t sound desperate or too-cool-for-school, like a simple “Hello” or “Hi, Mr. [name] or Ms. [name].”
Use careful judgment when using the first name. If the email is going to a CEO of a major corporation, it’s best to stick to the last name and say, “Hello, Ms. [last name].” But if you’re sending the cold email to a freelance graphic designer to solicit his/her work, it may be suitable to address him/her by their first name.