Honestly....you already have them. Every single person in the world HAS abdominal muscles. The difference is that some are very visible (like mine before), some are partially visible (like mine now) and others aren't visible at all. So the real question is, how do you make them appear? Is it from exercise or diet?
I am a true believer that 'abs are made in the kitchen'. Meaning that if you diet right and lean out, you will eventually be able to see what you have beneath the layers of fat. However, will it be the abs you want? Maybe...maybe not. And just how lean might you have to get in order to see them? Some people might need to become underweight in order to see anything! I, for instance, was 120lbs in order to see my abs in the photo above... and I'm 5'7" - that's too lean for a daily weight.
And that's where training comes in...
Weighted, body-weighted and other more traditional abdominal exercises are what will develop the core into the work of art you desire. By training the abs, you can 'thicken' them so that they become more prominent and 'stick out' more, which means you don't have to get as lean in order to see them. Bonus! (this is what I need to do for the future.)
Additionally, with training, you can shape your core to your liking. Maybe you wish to make your obliques larger. Or maybe you want to make your upper or lower abdominals the focus...it's entirely up to you.
So really, the answer depends entirely on what you already have underneath the layers and what sort of look you desire. And that is why I STILL say abs are all about diet. Because ultimately, in order to see any of your hard work, you're going to have to diet down at least a bit in order to see them.
But if you want the best of the best results, diet down and train them at the same time! That way you know that when you are lean enough to reveal them, they'll look exactly how you want them to.