1) Low body fat is most important. It doesn't matter how hard you work your abs if they are covered by even a relatively thin layer of fat. This is also the most difficult part. Many people simply don't have the body type to get down to the body fat % level necessary without completely overhauling their lifestyle. Generally, you need to be below 10% for good definition and that ain't all that easy if you don't have the body type.
2) As such, cardio and diet are most important to get body fat % down.
3) Here are some exercises to work on those abs;
Saxon Bends-Good to stretch your abs
Hanging Leg Raises-Good for those lower abs (but it is a bitch to do)
V-sit ups on wall-nice and uncomplicated, but be sure that only your heels are touching the wall and legs are straight
Hanging Knee Tucks-great for abs, biceps, and shoulders but is VERY difficult, but if you can do a full rep of 20 you will be the envy of the gym.
Alternate knee cross-ups-nice for when you want some variation, but can be tough on the back.
Hanging alternate twisting knee tucks-only do these if you have a good strong grip.
Leg swings-great for those lower abs and gives your arms a rest if you've been doing some hanging exercises
4) Don't get too narcissistic about it. If you just want to show off your abs, then I'd suggest not turning your life upside down for it. It isn't worth it. I've known people who went on near starvation diets and spent hours and hours at the gym just trying to get better abs. As I said, it isn't worth all that.