Picking Recommendation Letter Writers - PhD Statistics

[Kalculus]

I am in a bit of a conundrum on which professor to ask for a letter of recommendation to apply for a PhD in Statistics for Fall 2024. One LOR writer is fixed; he is a well-known statistician and I am fortunate to be endorsed by him. For the other two LOR writers, I could pick from among my undergraduate thesis advisor, my measure theory lecturer, and a lecturer with whom I have taken a lot of math courses including linear algebra.

The latter two know me very well. However, they cannot attest to my research potential which I assume statistics admission committees would be looking for. On the other hand, my research is only partly complete, it is on a mathematical topic that has little connection with statistics, and I have not taken many courses with my thesis advisor. Nonetheless, I believe her letter would be good, since she has commented a few times on my progress so far being admirable. Is it essential/advisable to have a letter from your thesis advisor? Which two LOR writers should I pick? Thanks in advance for any advice. 

[Kalculus]

P.S. My grades for the courses I have taken with the latter two lecturers are all very good. Their letters are certain to be excellent. 

[BL4CKxP3NGU1N]

I would pick the undergrad thesis advisor and probably the lecturer you've had multiple courses with, assuming you have a good relationship with them.

Very few undergrads have done any meaningful statistical research, so I don't think an admissions committee would expect that of you. Your undergrad advisor will be able to attest to your general research ability and potential though, which is definitely important. It also sounds like you have a good relationship with them and have done some good work, so it's a no-brainer to me to pick your thesis advisor.

Between the lecturers, it probably doesn't matter much which one you pick as long as you've developed a relationship with them and they can attest to your good work and ability in the classroom.

[Kalculus]

Thanks for the feedback! I just assumed that measure theory would be a big asset in applying for higher-ranked statistics programs. My grade for measure theory will not appear on my transcript since it was a recent course. However, the lecturer may be able to talk about my performance in his letter.