Achieving net-zero emissions across global power systems requires an integrated planning framework that jointly considers generation, transmission, storage, and demand-side flexibility under deep decarbonization constraints. This study develops such an integrated planning model and applies it across major regions to identify least-cost pathways toward fully decarbonized power systems. The results highlight the central role of renewable expansion combined with cross-regional transmission, long-duration storage, and flexible demand. The analysis offers actionable insights for policymakers designing power-sector decarbonization strategies that are technically feasible, economically efficient, and equitable.