Use these two exercises as a progression from the ab wheel. The videos don't do them justice. You've got to try them to understand it.
Keep these in a low to moderate rep range, multiple sets of 5-8 reps. Going high rep on them can make the low back kick in. But you've been warned: This will leave even veteran lifters shaking like a leaf on a tree.
Mike Boyle hates the question, "If you could only do one exercise, what would it be?" To him, it's like asking if Superman could beat the Hulk. Regardless, he gave us an answer that's pretty intriguing.