I had a number of people to choose from when it came to asking for LOR's. I chose the ones that knew me in the capacity I wanted to focus on. For instance, one prof is very familiar with my situation of being a non-trad, and I know will write a very heartfelt letter about my maturity, attitude, and personal strength as a full time student, mother, etc. Another is a prof with whom I've had several conversations with and I always go to her with questions about everything. Her letter will focus on my enthusiasm, intelligence, etc. And of course, my research supervisor will focus on the aspect of me wanting to be a DO/Ph.D.
My advice is to think about how each prof knows you, and choose those strengths for them to write about so when they write a rave review for you, it will sound genuine because they are writing about the capacity in which they know you. I also supplied my profs with copies of my PS, a little background sheet about my life, family, stats, classes I've taken, goals, work experience, philosophy of medicine, and on and on...