What is the best ab exercise for asthetic results?

Considering that you do have a healthy diet and that you are including intelligent cardio (love that Joy!) into your workouts, I would suggest incorporating weights along with your abdominal exercises.
I have found that by adding weights to your exercises, you tax your muscles with more emphasis and speed up your results plus it is less boring.
Less boring because you get to use a variety of different equipment such as dumbbells, weight plates, medicine balls, barbells, etc…..
I would include weighted saxon bends, bicycle crunches, and russian twists.
I personally follow up with the best abdominal strength exercises (in my opinion)
the forward and side plank.