We report very efficient spin-current generation by the spin Hall effect in the alloy Au0.25Pt0.75, which, as determined by two different direct spin-orbit torque measurements, exhibits a giant internal spin Hall ratio of ≥0.58 (antidamping spin-orbit torque efficiency of approximately 0.35 in bilayers with Co), a relatively low resistivity of approximately 83 μΩ cm, an exceptionally large spin Hall conductivity of ≥7.0×105Ω-1m-1, and a spin diffusion length of 1.7 nm. This work establishes Au0.25Pt0.75 as a milestone spin-current generator that provides greater energy efficiency than that yet obtained with other heavy metals or with the topological insulators Bi2Se3 and (Bi,Se)2Te3. Our findings should advance spin-orbit torque-based fundamental research and benefit the development of fast, efficient spin-orbit torque-driven magnetic memories, skyrmion and chiral domain wall devices, and microwave and terahertz emitters. © 2018 American Physical Society.