Over the past decade I have been in the weight room I have done countless crunches in my day. I tried legs lifts and side bends and decline sit-ups with a 45-lb weight above my head, and just about every other exercise I could think of. I wanted a 6-pack. After years of endless crunches and so-so results, I came across the works of Dr. Stuart McGill and changed how I train my core forever. This exercise is a great way to work on anti-rotation; the main functional of the core is not to initiate movement, but instead to stop movement. The Pallof Press is an awesome exercise to work the core in a functional way and get great results.
Use a resistance band, or a cable pulley at mid height, and warp it around a stationary object. The farther away you are the more difficult it is going to be. Also you can make it more difficult by having your feet closer together. Focus on standing tall with slightly bent knees and chest up, then extend your arms. Don't allow any rotational movement and then bring your arms back. This is an good beginner to advanced exercise.