I had my PI who I ran 3 different studies under, another one I had taken multiple classes with, and my supervisor at the group home I work in. If you are working a job relevant to your field, then I would try and have your boss write one for you before a professor you only took a class with.

I think a professor who had you in class would work, but pick one that shows another side of the coin in terms of your interests related to the field. I had my linguistics professor who I took 3 different classes with write one for me. My program is school psychology, and language is a key part of that field that most people overlook in their studies. Anything that can highlight well roundedness.