QM, with impeccable form, on a wobbly rail, for as long as you can. Alternatively, walking in a plank position while holding a wheel between your feet. Or loading heavy sacks of cement mix onto a truck for a day. Or saut de bras and climb-up drills. Or walking on a narrow rail with a weight across your shoulders.
The "core" is not a single body part but pretty much all the muscles of the torso working in concert to stabilize the torso while the limbs engage in movement. People think the core is just the abs and it's not. It's everything from pretty much your hip sockets/lower buttocks on up, across the back, the abs, the chest, and the shoulder girdle. "Core strength" is really more a function of stability and muscle stamina rather than "strength" per se, and it originates in the pelvis.
Any exercise that forces you to hold your torso stable while the limbs move will train your core; more so if the moving limbs are also bearing weight.
If you want a complete exercise system that will give you a solid core, try ballet.