What is the most efficient model of a submarine?

Until the advent of Nuclear Power for submarines, which gave them much greater speed capability underwater than on the surface, submarines were essentially designed as low-profile surface vessels with limited submergence capability. As such, their overall design made them noiser underwater and faster on the surface rather than quieter and faster underwater, the hallmark of modern submarine designs.

The most hydrodynamically efficient model of submarine is the elongated teardrop shape that is characteristic of most large modern military u-boats. These boats have a rounded nose, a hull diameter that is appropriate to the design objectives of that particular boat, and then the hull tapers down in diameter back past midships to just a few feet around at the aft end where the screw is attached to the shaft. It makes for really good hydrodynamic performance and good efficiency. It's a quiet shape, too, and it's all about the noise, isn't it? Sharp points on the bow make for extreme difficulties in design and add little to nothing to the performance of the hydrofoil. (It's more about cross-sectional area after considerations regarding the shape of the nose, or bow.)

Also, as mentioned, a nuclear submarine is operationally efficient because it can operate at sea for extended periods without the need for fuel, and, more importantly, can operate UNDER the water for a long time because the propulsion system doesn't require air - oxygen - as part of its power generating cycle. Lastly, think about this: an old diesel electric submarine, when it is operating underwater using its battery banks to drive an electric motor to turn the screw, is REALLY quiet. A nuclear boat constantly runs pumps and steam turbines while underwater, and these actually make more noise than the electric systems that drive the old boats when they are submerged. Of course, when the old boats surface to run engines to charge batteries, they can be heard for many miles and identified for what they are.