One of the toughest and most effective core exercises I’ve ever done is know as the TRX scorpion plank. This thing is brutal if nothing else because it takes the effort from a plank, and adds a leg and arm rotation, all the while being suspended by the TRX.
Any exercise for the obliques! I feel they are the most satisfying. A few favorites include the side-ups in the short box series on the Reformer, the pull-ups with a twist on the Chair, as well as all twists in the side-arm series on the Reformer. Very deep work