Increasing dampinglike spin-orbit torque (SOT) is of fundamental importance for enabling new research into spintronics phenomena and also technologically urgent for advancing low-power spin-torque memory, logic, and oscillator devices. Here, we demonstrate that enhancing interfacial scattering by inserting ultrathin layers within spin Hall metals with intrinsic or side-jump mechanisms can significantly enhance the spin Hall ratio. The dampinglike SOT is enhanced by a factor of 2 via submonolayer Hf insertion, as evidenced by both harmonic response measurements and current-induced switching of in-plane magnetized magnetic memory devices with the record low critical switching current of approximately 73 μA (switching current density of approximately 3.6×106A/cm2). This work demonstrates a very effective strategy for maximizing dampinglike SOT for low-power spin-torque devices. © 2019 American Physical Society.