One of the best core exercises is the plank. It is also hard to cheat at this exercise, making it a true test.
Works: Whole core, especially your rectus abdominals and transverse abdominals, as well as your erector spinae on your back and glutes if you engage them.
Position yourself like a table on elbows and feet. Keep your back straight and hips level. Hold this position for 20 seconds and try to keep a solid position.
To take it up a gear, try lifting one leg up off the ground for 10 seconds, then swap legs. Keep the hips level while one leg is lifted.