This letter investigates the optimal sensor placement strategy for single static target localization using the hybrid received-signal-strength (RSS), angle-of-arrival (AOA) and time-of-arrival (TOA) measurements on the 2-dimensional (2D) plane. Firstly, the problem formulation is introduced. Secondly, the theoretical smallest trace of the Cramér-Rao lower bound (CRLB), which is equivalent to the smallest estimation mean-squared-error (MSE), is derived using the A-optimality criterion. Besides, we propose that the reachable smallest trace of the CRLB (tr(CRLB)) will be determined, once the number and characters of the sensors are selected. Thirdly, the corresponding optimal sensor placement strategies are proposed and concluded in two observations. Finally, the findings are demonstrated by simulation examples.