Abs exercise will not give you six-pack abs. You need to do compound exercises and intense cardio such as interval training. Also, it is much more important to eat right than do sit-ups everyday.
I use powerlifting compound exercises like the power clean and press. For specific instructions on how to do this move just google it or watch on youtube:
YouTube - 240x11 clean and press
It works out almost all the muscles which lets you burn fat faster. It is bodyfat percentage that determines whether you have six-pack abs or not.
In simple terms:
Six pack abs = Eating right (low carbs, high protein/fat), interval training, compound lifts (clean and press, deadlift) then finally core training (sit ups, planks, leg ups) so as you can see, abs specific exercise is very low in priority and in my opinion is not even absolutely needed to get a six-pack to show.