Sorry, adding to above.
For examples (they tend to help sometimes...):
1. Chief of orthopaedic surgery who I 1. babysat for as a teenager, 2. volunteered with/shadowed for a summer, 3. who I was hired to do research with for 2 years and who I published with.
2. My thesis advisor from my masters program who I created a course with, did my masters thesis under, and who I TAed for for 2 courses.
3. One of the most famous international MDs who I worked with (him and his team) in my research work in Haiti.
4. My undergrad advisor who I kept in touch with and became friends with over the years.
I think CONSISTENT and diversity of contacts is important. I think you need to make a list of all the possible LOR writers you have in mind, then list how they know you, what they know about you and the strengths they each could say about you. Then pick the most diverse group that shows all sides of you, and invite them. I sent to every LOR writer a copy of my AMCAS (rough draft), an outline of our relationship and what I thought they could say about me (i actually drafted LORs for each of them as well), and it was very well accepted by all 6 people I ended up asking.