There are some good comments on the importance of LORs here:
http://forums.studentdoctor.net/threads/are-recommendation-letters-even-important.1174545/
It's hard to really quantify HOW good a LOR needs to be. My guess is that most people probably have good but somewhat average letters that won't hurt them but also won't be the deciding factor in their acceptance either. A truly stellar LOR probably comes from someone who has truly stellar ECs.
Just my .02 on good people to get LORs from:
--Science profs who remember you and can speak to your abilities in class (Many schools require 1-2 letters from science profs)
--Supervisors from a clinical, leadership, or volunteer experience (Someone who can attest to your understanding of what it means to be a doctor, qualities you possess that would make you a good doctor, your altruism, your motivations, etc.)
--Work supervisors, if you have them (not everyone will)
--Research supervisors, if you have them (not everyone will)
Ask people who know you, who you have good relationships with, and who can attest to your abilities academically and professionally. Your LORs give adcoms insight into how others (ideally those who know you well) perceive you. If you've been doing well academically and in your ECs, it shouldn't be too hard to find good letter writers.