Following is the preferred order of sources, in the eyes of a potential employer:
The one major exception to this is when your personal professional acquaintance is also connected with your potential employer (yes, it's called "pulling strings" and it's done all the time). Then it goes to the top of your list. Please leave off personal acquaintances (one candidate I interviewed listed his mother as a reference, saying, "Who would know me better!") and religious acquaintances (five years as an altar boy is not typically considered a measure of employee loyalty).