Which chin/jaw procedure is best for a flat profile with a long chin?

Without an exam your question cannot be answered.  You will need to see an Oral/Maxillofacial surgeon to see if othognathic surgery, genioplasty or using an implant is best for your specific case. It should be noted that patients with severely weak jaws or abnormally functioning jaws may be better treated with other facial surgery (orthognathic surgery), although a chin augmentation may be an option at a later time. These patients should seek the advice of a plastic surgeon to determine which course is right for them. Here is some information to help you make this decision: Candidates for Chin Augmentation - Patients whose chin profiles are weak but have normally functioning jaws are candidates for Chin Augmentation. Many of these candidates have used injectable fillers in the past to strengthen the protrusion of their chins, but are looking for a permanent solution. If you are one of these patients, then you are a likely candidate for chin augmentation
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What I am seeing on the photographs is a weak midface giving the illusion of a longer face. To achieve better facial balance, you would need fullness in your midface.  This can be achieved through soft tissue filler or implants.  However in your case the projection necessary to improve for facial proportions would be more than filler can accomplish. I would recommend malar implants.

Your chin is vertically long and your midface is flat. A combination of a vertical reduction and narrowing intraoral chin osteotomy and premaxillary-paraanasal augmentation would address both facial issues concurrently in a single operation..