For breast which is best digital mammogram or sonogram?

Depends, maybe both: A digital mammogram is the standard for mammograms. Sometimes an ultrasound may be added to get more information, especially if you have dense breast tissue. In younger women with dense breasts, a mammogram may no be done and then we go straight to ultrasound. A breast radiologist and breast surgeon will be able to make the appropriate recommendation for you.

Mammo vs sono: Mammography is still the screening standard of care for dense breasts. The sensitivity of mammography for detecting cancer(80% across the board) decreases in dense breasts, some studies show to 50% or lower. There is some data to suggest sonography performs better than mammo in dense breasts. Many are now performing both screening mammo and sono in dense breasts, it is an evolving situation.