Frog Kicks/Hanging Leg Raises. Those or reverse crunches. Try to train the entire core in the following order, though:
TVA (activated first, this is your 'inner ab wall', the transverse abdominus)
Lower Abs (recruited second in movement)
Obliques (third)
Upper Abs (Take a guess...)
You can work the TVA through Bridges and Vacuums.
The lower abs through the exercises I listed above.
The obliques through russian twists, side bridges, side bends, or medicine ball work.
Upper abs can be trained with the crunch, but do it on a balance ball for the full ROM.