When it comes to training abs, most exercises are straight-up boring and some can be downright painful, especially if you're not used to doing direct ab work. Here's one big exercise that uses continuous tension and heavy resistance, and it puts a special emphasis on the serratus (those awesome-looking "finger" muscles where your chest, lats, and abs meet).
Begin the low cable serratus crunch similar to a basic crunch, lying on the floor, with your head near a low cable pulley. Grab a straight bar and keep your arms locked and perpendicular to the ground the entire time. Perform a regular crunch, but at the top of each rep, actively press your arms even further towards the ceiling. This is what activates the serratus in a big way.
The key point with the movement is to extend the arms straight up towards the ceiling, not forward in front of your chest. It's also important to reach as high as possible at the peak of each repetition and hold that top position for a full one or two seconds.
Using 2-3x10-12 with this exercise will be plenty. Be sure to keep your arms locked straight and pointing up towards the ceiling throughout each rep, use a full range of motion, and squeeze the serratus at the top of each rep. You should feel a solid contraction in the upper abs, just below your chest. That's your serratus, and they're going to be sore later.