Atomistic simulations of electron and phonon transport are performed within the non-equilibrium Green's function formalism to analyze the thermal and electrical properties of graphene nanoribbons (GNRs). We predict that by patterning GNRs properly, a strong enhancement of thermoelectric properties can be achieved. From the study of edge orientation effects, we propose a strategy likely to degrade the thermal conductance while retaining high electronic conductance and thermopower. An effect of resonant tunneling of electrons is evidenced in mixed GNRs consisting in alternate zigzag and armchair sections or in perfect GNRs with vacancies. Combining these effects, an optimized structure able to provide a high thermoelectric factor of merit ZT exceeding unity at room temperature is demonstrated.