This paper considers the best channel selection for reduced-dimension space-time adaptive processing (STAP) in the presence of multiple targets. An algorithm based on semidefinite programming (SDP) is proposed to achieve the best channel selection by optimizing the worst case of the detection performance of the different targets. Compared with some existing algorithms which only consider the single-target case, the proposed algorithm can provide considerable performance improvements for detecting several different targets. Simulations are conducted for validating the proposed method and demonstrating their high performance.