Dips because it works out both your triceps and lats. To increase the intensity of the dip, try with one leg up. To go even further, use two chairs -- your hands on one and feet on the other so that you have a deeper range of motion. While you can increase the amount of weight you use for skullcrushers/tricep extensions, it's an isolation move that only works out one muscle at a time. Compound moves are more efficient, time-wise.
