You'll be hard-pressed to find any exercise that hammers the anterior core like the ab wheel rollout. If you can do 20 rollouts while keeping a posterior pelvic tilt (glute squeeze) throughout the set, you're a badass.
This exercise heavily activates the upper and lower rectus abdominis and the external and internal obliques. In fact, it's so effective that it can be dangerous if you jump right into it without proper progression. Get in a couple of bouts of RKC planks prior to starting ab wheel rollouts and don't go overboard on the volume.
Rollouts from the kneeling position are highly challenging and they'll work the core musculature even harder if you squeeze the glutes, which produces a slight posterior pelvic tilt and prevents the abdominal wall from being stretched during the exercise. This is important because the ab wheel rollout is an anti-extension core stability exercise that strengthens the spine's ability to resist hyperextension.
Some advanced lifters can even perform them from the standing position. Try it if you dare. No ab wheel? Some lifters make a barbell work for them.