I agree with what he is implying. In actuality, if you are performing a large variation of compound exersizes, you will not need a lot of ab-isolation work. Compound movements such as squats, bench presses, rows, and deadlifts utilize your abdominals as stabilizers. However, if you must isolate here are some good ones:
Cable Crunches
Decline Crunches
Hanging Leg Raises
Swiss Ball Crunches