Reveal abs
Lots of people think you do have to do crunches for having 6-pack abs. Well this is WRONG!
To reveal abs you don't have to do any exercise. The most important thing is dieting and reducing fat. Everyone has abs, it is hidden under a layer of fat. Cardio will help to reveal it.
Dieting and dedication is the most important thing to have abs.
It is healthy to achieve 2 gram protein (chicken, steak, fish, cottage cheese, ...) per kg bodyweight and 1 gram of fat (healthy fats) per kg bodyweight. Fats from olive oil, fish, unsalted nuts. The rest of the calories do you have to gain from carbohydrates for energy. AVOID SALTS AND SUGAR. And one last tip, drink enough water. Try to drink atleast 2 liter water per day. More is better.
BUT
Exercising abs is still good for you. It strengthens your core and reduce lower back pain. I still recommend some ab exercises for making your core stronger. If you have already revealed abs i believe exercising makes them look even better.
Exercises
Plank
Side plank left & right
Lying leg raises
Half Burpees (begin in plank form and jump towards your hands and jump back)
Bicycle crunches
Hanging Knee raises
Hanging leg raises
Hanging side raises
L-sit pull-ups
Toe touch crunches
I personally recommend Hanging knee/leg raises. You really feel it burning and it doesn't get easier. Challenging exercises! This will take some weeks for actually seeing abs. Challenge your day, every day!