Heavy metals and topological insulators are promising materials for converting charge current into spin current for efficient manipulation of magnetization states in magnetic devices -. One of the most important parameters is the charge-to-spin conversion (CS) efficiency. Improving CS efficiency is critical for reducing write current of the emerging nonvolatile memory technology, spin-orbit torque MRAM (SOT-MRAM) , which provides comparable speed with SRAM but with a much higher memory capacity. Here, we measure CS efficiency in various topological insulators (TIs) using second-harmonic method (2eo-method) and obtain a record-high value 8.33±0.65 for insulating (BiSb)2 Te3 at room temperature. We first establish the consistency of CS efficiency obtained between spin-torque ferromagnetic resonance (ST-FMR) and 2eo-method. Then, we systematically investigate the CS efficiency in a bilayer consisting of a metallic Bi2Se3 and a CoFeB thin film using 2eo-method. By tuning the Fermi level of TI layer into bulk band gap using (BiSb)2Te3, we improve the CS efficiency by an order of magnitude.