If you are looking to build a strong core foundation then you need to focus on large muscle group compound movements such as squats, deadlifts, rows, and standing presses. A 2013 systematic review, which is the top tier in research design, found that compound movements were superior to core-specific exercises in training and strengthening key core muscles. Additional studies have also shown high core muscle activation with movements that require upright posture while holding weights such as standing shoulder presses and weighted carries. For examples of great compound movements I would suggest checking my post on the best functional exercises here.
Now you may be asking, "What if I just really want to focus on the six pack muscles?"
Well, first I would tell you that just focusing on that muscle is not the best strategy. While the six pack muscles may look good, they don't really do much for core stability. That being said, as long as you are training the core with various compound movements then I have no problems with a little special attention to the six pack.
So what are some exercises you can do to strengthen the Rectus Abdominis? What is that muscle you ask? That is the six pack muscle. It is the big one that lies on top of the other abdominal muscles and gets all the attention. And there are a few exercises that you can do to make this muscle bigger, stronger, and more visible.
One 2014 study compared 3 anterior chain whole body linkage exercises and found that they all produced high activation of the abdominal wall with significantly high levels for the rectus abdominis. Additionally this study found that all three of the exercises were quite conservative in terms of spine load compared to other posterior chain exercises that use an external load. That simply means that these exercises can provide you with a great stimulus for increasing abdominal size and strength while conserving your back. Here is a video demonstrating each one of the exercises mentioned in the study plus a few extra that I think are equally effective.
According to the study the Leg Raise created the highest activation of the rectus abdominis compared to the other exercises. In second place was the Walk Out followed by the Body Saw. The TRX Rollout and the Ab Wheel are essentially different variations of the same exercise and are also very similar to the Walk Out.
If you are strength training three days per week, which I typically recommend, then you can focus on just one of these exercises each day you train. I would start out doing as many good repetitions as possible and perform three sets. Gradually work your way up to 10-15 reps for reach exercise. Focus on staying tight through the abdominals and lower back and go at a smooth and steady tempo. Do not rush through the exercises just to count reps. If you can work your way up to 3-4 sets of 15 reps for each exercise, and get your diet in check, then you should have a pretty nice midsection. Personally I perform each of these exercises on a regular basis, I do strength training 3 days per week, and I watch my diet. I turn 30 this year and so far I am happy with the results.