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.
