"Hello" or "hi" — what's best?

I believe Hi is less formal than Hello, But if you want to know when to use what ,I prefer Hello for the phone, letters and Hi in face to face conversation or gestures and Instant messages .Rest is really your choice.

I remember one time I used Hello in that email at office and my senior advised me to use Hi. I believe he didn't have the reasonable explanation but due to culture of the office he said so. What I understood from this is that it also depends on the culture of the particular place.

Hi is slightly too informal for an introductory email.

This is a matter of personal preference and style. To my knowledge, the following seem to be acceptable usage:

Hello, Jim. or Hello Jim,

Jim,

Hi Jim, or Hi, Jim.

Dear Jim, or Dear Sir,