Screw planks. Do this exercise to take your core strength to the next level.
Nothing builds a strong midsection like doing exercises where you stand erect with a heavy weight. That's why my "core" training includes this move:
Zercher Holds from Pins
Zercher holds will absolutely kill your midsection. I've never felt intra-abdominal pressure to the degree that I feel on this exercise. I do a quarter Zercher squat and hold the position for 6-9 seconds.
You must tense your abs super hard. Imagine you're about to get punched in the stomach. Squeeze your glutes as hard as humanly possible. If you don't, your lower back will take too much of the load.
Don't lean back even one bit. Doing so not only makes the exercise worthless, it also makes it dangerous.