Two of the nastier but highly effective core exercises I do:

1) Hanging from rings or blocks (also works from a pullup bar, but something that moves makes it more challenging) slowly bring legs up past parallel hold and slowly release. No movement, swinging, etc except the movement of your legs. Repeat until collapse. When I started a could to 7 or 8 with control but no pause at the top. Now up to sets of 15 with 2-3 seconds pause at the top of each one.

2) Lying on your back raise your right leg to 90 degrees and hover your left leg about 6" off the floor. 25 crunches each side, keeping your legs as still as you can and reaching through the balls of both feet.

I mix in a lot more, but those two are killer for me.