Honestly, for the majority of people, actual ab exercises are the least important thing when it comes to having a good 6-pack. Everyone halfway educated about training says abs are b uilt in the kitchen. That's not entirely true, because it doesn't matter how much fat you get rid of - if there's nothing underneath it you could have 5% bodyfat and no visible abs.
But in saying that, nutrition is an important factor, because the other side of the coin (the side that the "abs are built in the kitchen" crowd are very aware of) is that you won't have visible abs if there's much fat on top. For most people, the lower abs are the last place to lose fat, so for most people they'll start with a visible 2-pack, the 4-pack, then, when guys get below 10% bodyfat, that's around the time they'll get their 6-pack showing. You won't need to do any special ab exercises to achieve that, you'll just need have a generally strong core and a low bodyfat.
The other key ingrediant (other than nutrition) is general strength training, and building muscle all over. Having a pretty 6-pack is all well and good, but you'll never have a great-looking 6-pack if it has skeletor as its frame. Did you used to watch That 70's Show? Imagine the main character, Eric Foreman, if the only thing that changed about his physique was that he had a 6-pack. That mental picture is more funny than impressive.
So instead of worrying about building abs specifically, worry about getting strong in deep squats (hips below knees), hinge movements (deadlift and Olympic pull variations), presses (especially overhead), rows and pull ups/chin ups/pull downs. Getting stronger in these exercises will give 99% of people all the core work they need, and will properly frame the core.