IEEEIn this paper, we present an optimization study for the terahertz Spoof Surface Plasmon Polariton waveguides (THz-SSPP WGs) for the first time in the literature. The optimization study employs a selection method for the optimum dimensions for the THz-SSPP WGs, which targets maximizing the insertion phase and minimizing the insertion loss levels, and hence, finding the optimum set of dimensions that result in the best ?/dB performance. The method first utilizes the analytical examination of the dispersion diagram to estimate a target range of dimensions for minimum insertion loss, which is then scrutinized by numerous simulations of the THz-SSPP WGs within the selected dimension range. In order to verify the optimization method, a set of THz-SSPP WGs are designed using the proposed method, fabricated, and measured in the 0.22-0.32 THz band. The measurements result in a record-low insertion loss per unit length of 0.75 dB/mm with the optimized THz-SSPP WGs. Additionally, the utilization of the optimized THz-SSPP WGs to compose a terahertz phase-shifter enables a record-high figure-of-merit (FoM) of 102.8 ?/dB, which is the most fundamental performance measure of the terahertz phase shifters. It should be indicated that both insertion loss per unit length and FoM levels make THz-SSPP WGs superior to the state-of-the-art planar terahertz waveguides. The presented method provides the most concrete evidence for the potential of the planar SSPP WGs for the terahertz band.