Students applying for PhD study, particularly in theological and religious studies, might find the following advice useful as they decide from whom to solicit letters of reference.
Those with confirming or dissenting advice are, as always, welcome to offer it below.
You, the student, want the ideal combination of (1) the highest possible praise (2) in the most detail, (3) especially about the most advanced (= independent & extended) work (e.g., a thesis or a comprehensive examination or a large research paper), from (4) the most prestigious ((a) in terms of the field or (b) in general) professor with (5) the best connections to the school to which you are applying.
I would rank them then in this descending order of importance:
(1) is non-negotiable. If you didn’t get an “A” from him or her, don’t ask for a reference.
(5) is next most important, since everyone else’s letters will be strong, but only some will have an “inside track.”
(4a) is next, then (4b).
While I’m at it, if there is anything odd (or unimpressive) in your record (e.g., a bad course or semester) to which a reference can speak on your behalf, remind him or her to do so and provide all the information necessary to help him or her do so. You can offer the excuse yourself in your cover letter, but such excuses will be more powerful, of course, if rendered by someone else.
And one more thing: Schools in the US and some elsewhere ask the applicant whether she wants to waive her right to see the letters of reference submitted on her behalf. WAIVE IT. It seems counter-intuitive: Who wouldn’t want to see what other people write about him or her? But you’re counting on people to write nice things about you, and the niceness of those letters shines the brighter when the readers know that the writers were protected by anonymity. So waive it.