I do some different movements on a set of olympic rings(don't know the actual name of the movements), exercises like push ups with the rings low and your feet on your bench, a similar movement to a ab wheel roll out but just with the rings and chins/pull ups, knee ups, dips, general stuffing around on the dam things all work my core crazy on the rings. But I think the best movement to smash my core is heavy ass standing over head presses and front squats, they kill the abs for me.