Mammogram, ct, MRI and ultrasound tests for breast cancer - what is best?

Situational. For screening: mammograms are the most valuable. For diagnosis (palpable mass, mammographic density): ultrasound can be very helpful. Mris are the most sensitive test for breast cancer, but their prohibitive cost makes this unaffordable for large-population screening. It is best used in high-risk patients, those with very dense breast tissue, and for evaluating the extent of a known breast cancer.
Depends. These are all great tests to evaluate the breast tissue. The can be used in combination and provide different types of information. Deciding which to use depends on: age of patient, density of the breast tissue and the type of information needed. Ct is reserved to evaluate for metastatic (spread of cancer outside of breast) disease.
Start with mammogram. For the vast majority of individuals, work-up/screening should start with mammogram. Ultrasound may be ordered next. Breast MRI is used to screen for breast ca in certain high risk individuals or to complete the work-up of a newly diagnosed breast ca patient. Ct has no roll in the diagnosis of breast cancer - unless one is incidentally picked-up on a chest ct.
Probably MRI. As a stand alone test MRI is probably the best but these tests all supplement each other and their are benefits to using all when needed.