Crunches and sit-ups primarily target the Rectus abdominus which is the classic "6 pack" muscle. The crunches, sit-ups, and hanging leg/knee raises really target this area for maximum hypertrophy. The ab wheel is great for strengthening the Transversus abdominus otherwise known as the TVA. This is a muscle that lies underneath the rectus abdominus and is not viewed as an aesthetic muscle but a stability muscle. Planks, ab roller, and stomach vacuum really work this muscle.
To get that six pack look the quickest and most effective thing to do is lower your body fat as low as possible. Remember, there is no such thing as spot toning. This means even 20,000 crunches a day won't give you a six pack if you're at an 18% body fat. Those fitness models that you see on the cover of Men's Health and similar magazines are generally in the 3-6% range. This is not a really healthy percentage to sustain.
I have been bodybuilding and powerlifting competitively since I was 14 and I generally can see my six pack at 10-12% body fat. It's a lot easier to sustain and healthier too.