The best density of bone to achieve a minimal difference in the elastic modulus between a titanium implant and the residual alveolar bone is D2. D2 bone is characterized by a moderate amount of cortical bone with a reasonable amount of trabecular bone, making it an ideal density for achieving a stable bond between the implant and the surrounding bone. This density of bone allows for better stress distribution and reduces the risk of implant failure. D1 bone is very dense, while D3 and D4 are less dense and may not provide the best mechanical stability for the implant.