What is the most effective abdominal work-out?

Core exercises. The best way to incorporate core-strengthening is to incorporate it with other exercises. Most bicep, tricep, and shoulder exercises can be done on a medicine ball, thereby forcing you to keep your core stable. If you choose to isolate the abs, make sure your back is stable - don't rock with sit-ups for example. Hanging leg-lifts and even pull-ups/chin-ups are great ab work-outs.
Weight loss. Less fat makes abs more prominent. I like squats and planks for abs.