Different schools may have slightly different requirements, so you have to know what they are before you assign letters.
It would be nice if you had feedback from an advisor who had access to the letters to help determine their relative strength. Hopefully you have some idea of who could be trusted to write you a strong letter. As to judging letters by the credentials of the writer: it's my opinion that a lesser ranked person who knows you very well and thinks highly of you, who can write a letter based on a longer standing relationship and inject their insights into your personal characteristics would write a letter of more value than a high ranked individual who knews you briefly or superficially and who would write canned generic comments with no personal touch. If two letters are equal in perfection, THEN choose the more respectable writer.
