Before discussing a procedure we should really focus first your anatomy, and then consider your goals. In short, no your breast is not "saggy", it just looks that way because of the position of the fold. You have breast asymmetry, which is normal. The "sagging" breast appears to have an elevated inframammary fold (IMF). The two photos you posted are mirror images, so I cannot tell if it is the left or the right breast. In my practice I check for this by asking patients to raise their arms straight up above their head, which lifts the breasts and helps to visualize the position of the fold (IMF). My guess is that the breast with the larger NAC (the "saggy" breast as you call it) has the elevated IMF, so the shape of the breast is overall different and would be unlikely to be corrected with a mastopexy only as it doesn't change the position of the fold. Also, the level of your nipples is not that far off. I would also want to measure the distance from the nipple to the IMF and assess the compliance of the skin and breast tissue (how much this distance increases under gentle tension). More compliant tissue means that If your goal is improved breast symmetry only, then a small periareolar lift (what you refer to as a "donut", most likely) should suffice. Remember, though, it won't change the position of the fold so your "saggy" breast will still appear to be "saggy". If you were also considering increasing the size (with an implant), then I would recommend you consider bilateral breast augmentation with adjustment of the fold on the "saggy" side, as well as a peri-areolar mastopexy to correct the assymetrical size of the nipples and also ensure they end up even. I would reposition the fold through a peri-areolar incision and reinforce this with permanent sutures. Remember, it is totally normal to have asymmetrical breasts, and you should consider a consultation with a board certified plastic surgeon to discuss your anatomy and your goals (sometimes finding photos from other plastic surgeons websites is very helpful!). Try to avoid going into the consult with a procedure already in mind, and good luck!