How well a person knows you should take priority over level of seniority or HBS alumni status.
I would say the most common mistake I see in personal statements is when students choose essays that are too generic.
A lot of times, they forget that the topic is the foundation of the essay, and so you really need to choose something to write about that you feel passionately about.
Use your best judgment on who you decide to ask - there is no set formula for who should be your recommenders.
We know it is not always possible to have a direct supervisor write your recommendation – we would not want you to jeopardize your current position for the application process.