Antenna array configurations have significant influence on the radiometric sensitivity of aperture synthesis microwave radiometers. In this paper, we propose a minimum degradation array (MDA) for optimum sensitivity. First, the degradation factor (DF) is defined to characterize the effect of redundant spatial frequency samples formed by an array on the sensitivity. Aiming at minimizing DF, a simulated annealing (SA) based method is proposed to search for an MDA, which, combining with a concept of augmented maximum baseline (CAMB), can effectively locate the true global minimum of DF. Numerical results validate the effectiveness of the proposed method as well as CAMB in optimizing DF. Further, the lower bound of DF is discussed. Finally, simulation and experiment results demonstrate that the proposed method as well as CAMB is of significance in achieving optimum sensitivity.