Is the best way to fix my bad breast reduction a breast fat transfer or implants? (photos)

Hello and thank you for your question and sharing your photos. I believe you benefit from a revision surgery, to remove some of the redundant inferior pole skin at the fold in a horizontal fashion. Fat grafting and implants are both reasonable options. You should discuss the pros and cons of each with your plastic surgeon. Best,

You need a formal evaluation by a licensed plastic surgeon and expert in breast revisions to determine what is best for you and your goals.
Corrective breast surgery encompasses the correction of an array of problems that can occur after breast augmentation or other aesthetic breast surgeries. It is considered the most challenging aesthetic surgery of the breast and requires a range of techniques, including the use of acellular dermal matrices, fat grafting, internal pocket adjustments, and external skin tightening maneuvers. Although highly complex, I find this surgery fascinating as it puts to use all of my skills to solve unique issues. Some patients may be traumatized by their previous experiences, so strong communication skills are vital to understand a patient’s goals and to help them understand the kind of improvements we can achieve.
This type of surgery is extremely important for the patients suffering from deformities, and my experience with these complicated cases also helps me provide excellent results and minimize complications with more routine breast augmentations and mastopexies.
I suggest consulting with a board-certified plastic surgeon and expert in Breast Revisions. - Dr. Cohen

From your photos it appears that you would benefit from skin excision to help with the shape.  If you are wanting more volume an implant will provide upper pole fullness to the breasts.  Fat transfer will give you more volume, but the overall look may be different than an implant.  I encourage you to meet with a board certified plastic surgeon near you to discuss your concerns and goals for surgery.