The letter of recommendation will be the most important part of your application. Schools ask for a minimum of 3 LoRs from faculty or from people who can attest to your research capability. When selecting a letter writer, think of the following:
Does this person have a Ph.D.?
Does this person do research?
Is this person known outside of Bangladesh?
Does this person know me well enough to write a strong recommendation?
Can this person compare me to Ph.D. students in developed countries?
As harsh it may sound, faculty in Bangladesh do not do enough research. Even when they do, its not research that people outside of Bangladesh are aware of. For admission committees to think favorably of you, the profile of your letter writer is extremely important.
Your letter writer (s) should be someone whose courses you took and/or RA'd under and/or TA'd under. Additionally, your letter writer can also be your supervisors from a research organization you worked in.
Your letter writers should focus on:
your ability to be able to pass the qualifying exams of the Ph.D.
your ability to conduct independent research
comparing you to Ph.D. students they came across
given Bangladesh does not have a steady pipeline of Ph.D. students, admission committees do not have a frame of reference in evaluating a Bangladeshi profile. Thus, a letter writer who did a Ph.D. at UCLA (for e.g.), can compare your potential to the students they came across when doing their Ph.D.