Whats your favorite exercise to work your core?

One of my favorite, unexpected ways to work the core is to foam roll my legs for a long time, while keeping plank positions. In other words, keep the hips from sagging by really scooping the abs and pushing away from the floor with either the hands or forearms. The most challenging one for the core is rolling on the front and sides of the lower leg in a prone position with elbows on the floor – either by doing a reverse crunch, or by moving up and down with straight legs, thereby using a lot of lats and tris. When the roller is by the ankles, there is so much unsupported bodyweight between them and your elbows that keeping a good plank position is a real challenge. Of course, you’re also multitasking with the myofascial release. And the usually neglected lower leg area really needs it!