Slow light enables spatiotemporal manipulation of electromagnetic waves at the nanoscale and allows access to a plethora of nonlinear optical phenomena. Although the guided waves in plasmonic waveguides are known to inherently possess a slow energy velocity, their ultimate light-trapping performance remains unknown as the effect of the waveguide's shape alteration has not been considered systematically so far. In this work, we theoretically demonstrate a free-form optimized metal-insulator-metal plasmonic waveguide for light trapping that exhibits a quality factor several times higher than that of the conventional linearly tapered structures. The quality factor of the optimized waveguide saturates to the theoretical limit at a surprisingly short device length, which shows a nontrivial inverse logarithmic dependence on the material loss. The demonstrated design has a quality-factor-to-footprint ratio comparable to that of state-of-the-art photonic cavities.