I want to apply for Ph.D. positions next year. I have research experience of more than five years, so I am considering applying for research-based universities mostly. Now I am confused about who I should ask for recommendation letters? I already arranged one LOR from my department head, I am confused about whom should I choose from these four? All of them know me and they all will write good things I believe.
There is an adjunct professor who taught a course during my studies. He is now the head of a reputed university in my country.
There is an assistant professor from another university, under whom I did a project. She is affiliated with my current lab, a collaborator mainly. But I had to perform some experimental work for her project (published now, I am 2nd author), so I worked in her lab for six months, which is out of the city and I worked there as a guest researcher.
A third is a postdoc, mainly my collaborator or co-author and we published four articles, including one in the top journal in our field. He is doing his second postdoc (a research fellow) in a reputed US university.
Another one was my course instructor during my bachelor's, an assistant professor. I learned a few experiments from her (she was not my thesis supervisor but she was well versed in some experimental tests so she taught me).
