A 2010 study compared core muscle activation during Swiss ball and traditional ab exercises. Researchers found that the Swiss ball rollout and the Swiss ball pike were the most effective in activating the upper and lower abs and external/internal obliques. Combine the two exercises and do the pike rollout.
A comprehensive ab training program should include several different exercise applications to ensure your workouts are well rounded. But if you haven't tried the pike rollout, give it a shot.