Is skin tightening laser the best treatment for breast sagging?

Lasers do not tighten skin significantly if at all. Modest ptosis can be treated with a small breast augmentation. More significant ptosis might require a mastopexy.

Thank you for the question. Based on your description of your breasts, it is most likely that you will benefit from breast lifting plus/minus breast augmentation surgery. Lasers are not effective. You may wish to attach frontal and side view pictures of your breast ( along with goal photographs) with your next question form more precise advice.
You will do best by seeking consultation with well experienced board certified plastic surgeons. Ask to see lots of examples of their work and communicate your goals clearly. Patients who are considering breast augmentation/lifting surgery should understand that this combination surgery is significantly more complex than either one of the procedures done separately.  In other words, the combination breast augmentation / mastopexy surgery differs from breast augmentation surgery alone in that it carries increased risk compared to either breast augmentation or mastopexy surgery performed separately.
Furthermore, the potential need for revisionary surgery is increased with breast augmentation / mastopexy surgery done at the same time. This revisionary rate may be as high (or higher) than 20%.  
Personally, I find that the breast augmentation/lifting procedure to be one of the most challenging of the breast operations I perform, even compared to somewhat complex revisionary breast surgery. On the one hand, when performing breast augmentation/lifting surgery we are increasing the breast size with breast implants; on the other hand, we are reducing the breast “envelope” in order to achieve the breast lift. These two “forces” must be balanced as perfectly as possible in order to achieve the desired results. Removing too much skin/ breast tissue is problematic; removing too little breast skin/tissue can also be problematic. Remember also that patients presenting for breast lifting surgery and general have lost some skin elasticity/thickness making potential incision line healing problems and/or recurrent drooping/sagging important concerns to communicate.
The analogy I use in my practice is that of a thinned out balloon, being expanded with additional air, while at the same time removing some of the balloons rubber surface. I hope that this analogy helps patients understand some of the issues at hand when performing the combination breast augmentation/lifing operation.
To achieve a surgical result where the breast implant and breast tissue “come together” and behave like a single breast is one of my goals but can be difficult to achieve. Essentially, we are trying to create a breast implant/breast tissue interface that feels and behaves as naturally ( as a single unit) as possible. Generally speaking, making sure that the breast implant has some sub muscular and some sub glandular component ( dual plane) and tailoring the overlying skin/subcutaneous tissue/breast tissue as precisely as possible over the underlying breast implant is key.
Despite these efforts, breast implants are after all a foreign body that don't necessarily stay where we wish they would; therefore, breast implant related problems such as positioning ( too high, too low, lateral displacement etc.) can occur and may be a reason for returning to the operating room for revisionary breast surgery.I use a “tailor tacking” technique that allows a determination of what breast implant should be used to SAFELY produce the results the patient is looking for. This technique involves use of a temporary sizer and temporary “closure” of the overlying breast skin over the sizer. The use of the tailor tacking technique is very helpful. Breast lifting involves removal of skin ( and tightening of the breast skin envelope) while breast augmentation involves expansion of the breast skin envelope. These 2 forces are counteracting each other. Again, despite these efforts, breast implant and/or tissue/skin complications may arise causing minor or significant complications.
Generally speaking, it is difficult to achieve the “perfect” result with breast augmentation/lifting surgery, despite best efforts. Patients should be aware of the complexity of this combination procedure, achieve REALISTIC EXPECTATIONS prior to proceeding, and understand that additional surgery ( along with the additional recovery time, stress, expenses etc) may be necessary in the short or long-term. Patients should understand that the results of the procedure will not necessarily match aesthetically the results of patients who have undergone breast augmentation surgery only.
For example, some patients who wish to maintain long-term superior pole volume/"roundness" may find that this result is not achieved after the initial breast augmentation/lifting operation. An additional operation, possibly involving capsulorrhaphy, may be necessary to achieve the patient's longer-term goals ( with superior pole volume/roundness). It is helpful if patients understand that this breast implant capsule used to provide the support for the breast implant is not present during the initial breast augmentation/lifting operation. The capsule (layer of scar tissue) forms around the breast implant and may be a good source of supportive tissue during revisionary breast surgery, Including correction of breast implant displacement/malposition problems ( such as bottoming out, symmastia, lateral displacement etc).
Potential risks associated with breast augmentation/lifting surgery include infection, bleeding, incision line healing problems, loss/change of nipple/areola complex sensation, and blood flow related issues to causing skin or tissue necrosis. Poor scarring, pigment changes, areola/nipple asymmetry etc. are also potential problems. Again, patients may experience implant related problems such as encapsulation, leakage, displacement problems ( too high, bottoming out, lateral displacement, asymmetric positioning etc.), rippling/palpability of breast implants etc. Patients may also be dissatisfied with breast size, shape, and/or how the breast implants and overlying breast tissues “interface” with one another. Occasionally, a breast implant may even have to be removed and the patient will generally be “implant free” for several months at least. Obviously, this situation can be quite physically, emotionally, and psychosocially stressful to the patient involved.
Given the complexity of the combination breast augmentation/lifting operation and the greater risk of revisionary breast surgery needed, there are good plastic surgeons who will insist on doing the procedures separately. For me, if I see a patient who needs a great degree of lifting, who has lost a lot of skin elasticity, or whose goal is a very large augmentation then I think it is best to do the procedures in 2 stages (in order to avoid serious complications). However, doing the procedure in one stage does increase the risks of complications in general and the potential need for further surgery. This increased risk must be weighed against the practical benefits of a single stage procedure (which most patients would prefer).
Conversely, if I see a patient who requires minimal to moderate lifting along with a small to moderate size augmentation (and has good skin quality), then doing the procedure one stage is much safer. Nevertheless, the potential risks are greater with a 1 stage procedure and the patient does have a higher likelihood of needing revisionary surgery.
Having discussed some of the downsides and potential risks/complications associated with breast augmentation/lifting surgery, most patients (If properly selected and who are doing the operations at the right time of their lives psychosocially) accept the scars associated with breast augmentation/breast lifting surgery as long as they are happy with the improvement in contour, size, and symmetry. I hope that this summary of SOME of the issues surrounding breast augmentation/lifting surgery is helpful to you and other women considering this procedure in the future. The attached link may also be helpful. Best wishes.

A 'skin tightening laser' will do absolutely nothing for your breast skin - except possibly burn it or create pigmentation changes. Talk to a board certified plastic surgeon about a breast lift procedure to improve breast shape and skin appearance.