Help in prioritizing LORs? Which would be most helpful

[okletstry]

Sorry in advance for the long post. Also posted on reddit.

I am applying to psychology PhD programs (non-clinical) for fall 2019 admission and hoping to ask some former professors and others for LORs in the next week or so. In part to help myself think through my options, I wanted to open my thought process up to you guys. Below is a list of people I'm consider asking to some extent and how I know them/have worked with them, in order of who I feel carries the most weight:

1. PI of undergrad research lab: I worked in his lab for 4 semesters, wrote a senior honors thesis using some of his old data, he won an APA lifetime achievement award, endowed professor in the department at a top 20 university, co-authored a theory, 60 pg long CV, you get the picture. Didn't work too often with him 1:1 but he definitely really likes me, would love to accept me into the program at my alma mater (but probs not applying), advised me somewhat closely on my thesis, has said he'd write a rec, etc.

2. Senior Honors Thesis professor: I took a year-long course to help guide me through the process of writing my senior thesis and he was the professor for the course. Lesser known in the field but very well-regarded as a professor and mentor at the university. Did many thorough readings of and provided thorough feedback on my thesis, advised on grad school admissions process, etc. Doesn't know my academic interests super well (they've changed since I wrote my thesis), but knows my writing and motivation

3. Undergrad advisor: Director of Undergraduate Studies for Neuroscience (my undergrad major), knows me well personally, taught a lab class and another course in which we wrote weekly "popular press" science articles based on primary literature so knows my writing and academic aptitude well, we've talked frequently about my goal to pursue a PhD and the types of questions and research that motivate me to pursue a PhD. Doesn't know my research experience very well

4. Summer research experience PI: worked two summers (2015, 2016) for him, MD not a PhD and studies Alzheimers through basic biology research (western blots, IHC, etc.), really likes me and said at the time he could help with recommendations, but haven't talked much since. I think he'd still be willing to write if I got back in touch but I wonder if those experiences seem too outdated/tangentially related now

5. PI of lab I now volunteer for: well-regarded professor in human development/developmental psych, hasn't worked with me since I initially met with her but could maybe ask lab managers about my skills, most recent academic I've worked with. I haven't done any writing or data analysis for this lab yet, so she would just have to go off of my general data collection skills, motivation, interests, etc. Volunteering 6 hrs/wk

6. Current boss at work: I work as a direct care staff at a residential psychiatric facility for teens, my boss would be able to speak to my work ethic mostly bc there is no academic or research aspect to my job. I took the job hoping to see whether I wanted to go into clinical and the answer is no lol but it is a neat experience. Doesn't seem like a good fit for LOR to me but she's who I work most closely with at this point so thought she'd be worth mentioning.

Any help/advice in how to sort through these is much appreciated. I'm applying to programs that accept anywhere between 2-6 letters, so I would love to hear what people think as a "cut-off" if I were to submit only some of these (like do I only have 2 solid LORs and if I'm able to only do 2, I should stick to only 2? Or do I have 3-4 solid ones that I should include if I'm able?)

Final question: Is it appropriate to ask a PI to collaborate with a lab manager on the letter? I worked more closely with a lab manager/project coordinator in most labs, so I think they'd best be able to speak to my abilities but wouldn't carry the weight of a PI.

tl;dr: help me sort through my LORs and order them from strongest --> weakest with a cutoff of how many I should use if I could choose anywhere between 2-6. THANK YOU!

[PsyDuck90]

You want to pick the top 3 people who could write you the strongest letter. Since you are intending to go to a non-clinical psych PhD, I would say it would be those who can speak to your research abilities and future in academia. So, this can be looked at a few ways: who have you done research for the longest amount of time, or who have you done more complicated work for? For instance, if the choice is between a PI who you've done 4 years if data entry for or a PI who you only worked for for a year but helped develop a study and did complex statistical analysis, I would personally rank the 2nd as more valuable. 

I think it should be fine to have a PI and lab manager write 1 letter. When I was applying, my research advisor actually suggested she co-write the letter with the doctoral student who ran the research lab since I worked more closely with that person day to day. You may want to feel this out before you ask, as you never know what can ruffle someone's feathers!

[okletstry]

Thank you for the help! I've asked my undergrad PI and senior thesis professor (#1 & #2), but I'm still struggling with the third letter. I asked both of my recommenders for advice as to who my third should be, but would love if anyone else on here has input!