I am considering applying to competitive PhD programs this year, but I am not certain who/how to ask for strong recommendation letters. I am not even sure the following experiences will yield strong recommendations or if they indicate that I am unfit for the programs.
I have the following people in mind already:
A mentor I did a research internship with at a top CS school in a very well-known CS lab (PI with h-index 100+). During the internship, I think I did great work in the computer vision/deep learning space, considering it was my first experience in that, and we came close to finishing in 2 months and potentially publishing. After this, however, it went kind of downhill. I couldn't commit to finishing because I started school, but I essentially said I would but never kept my word, several times. Surely, this wouldn't look good. I still feel this mentor is supportive of me, but I don't know how the following experiences after that summer will impact my recommendation.
A professor at my school who I worked with briefly, also in the AI/ML space. I haven't published with them, but they are very committed to my success, were willing to take me in as a PhD student in their lab, and I feel confident they will give me a strong recommendation. They publish yearly and their lab is growing. h-index ~30s
I am only not certain of who to ask for the 3rd recommendation.
I have these options:
3-1) Renown researchers from an internship experience from freshman year at an Ivy League school. This was more using applied CS for the biological sciences, but these researchers are very well known. They've published in Nature, etc. Thing is, I don't think I left a particularly strong impression on them as a freshman. I didn't contribute anything extraordinary and even when I asked to extend the work into the year, I sensed they were a bit hesitant about it. One of these researchers did, however, provide me a recommendation for one of the future internships I got. This was years ago, and I have not been in touch with them since.
3-2) Manager/co-worker from my industry research internship. This was 2 years back. This manager essentially made it known to me that I was a stronger software engineer than a researcher. Neither the manager nor the co-worker are particularly well-known research scientists, but the company itself is. The work was in the NLP/AI space.
3-3) Another CS professor who was supposed to be doing joint work with me and 1) but never really got around to it. This person is Dean of the department at my former university now, but considering we never worked closely together, he wouldn't have much to comment on other than to iterate the work I did with 1). h-index ~30s
Out of the options for the 3rd recommendation, 3-1, 3-2, and 3-3 -- who should I ask? After all of this, I'm pretty much not sure if I should even pursue PhD, but I don't know if that's my impostor syndrome talking. I graduated with a 3.9, a great CV, and all these great research experiences. I am an underrepresented minority and my school wasn't particularly well known.
Any advice would be much appreciated