In my unqualified (read: not accepted... yet) opinion, you should have:
1) A committee letter. This is not opinion, but fact that every school requires a committee letter IF your school has a committee. If they do not, then the COM will list a substitute requirement, such as 2 letters from science prof & 1 from non-science prof, or 3 letters from professors.
2) A physician letter. Nearly every school requires a letter from a physician. Some require a DO, some strongly recommend a DO (read: get the DO letter, increase your chances), others will accept MD or DO. There was a good post on the SDN front page recently about what makes a good physician letter, I think. Get a strong DO letter to improve your odds.
3) An outside professor letter. The committee letter can be a wildcard school-to-school, as some will write "we recommend student A based on his history here because no professors had a problem with him and thought he worked hard" while others will compile glowing praises from your professors. As letters are confidential, you have no idea. Pick a professor, preferably science but potentially non-science, and ask them write about you, your desire to get into medicine, your positive attributes, etc. (Note that this backfired for me last cycle, as a journalism professor with whom I was very close wrote such a fantastic recommendation that two adcom members said the committee felt journalism would be a better career for me).
4) (If necessary) A boss/supervisor letter. If you have worked in healthcare or in any relevant field, ask your boss or supervisor to write a recommendation. If you haven't worked in healthcare, this letter may still be valuable, if it accurately and persuasively details your committment to medicine as well as any leadership or other positive attributes you have.
I believe the consensus is to keep it under 5 letters if possible. I've spoken with a few admissions people (through the phone, e-mail, and as patients at work), and they seem to agree. More than 5 becomes overwhelming and the true recommendations get lost in the noise. In my opinion, you can maximize the value of your 4-5 letters by following what I've written above.