The ab wheel is the best tool for core strength. For actual ab strength I just like to do a few basic exercises. Generally incline situps with weight, and cable crunches. Aside from the ab wheel for "core" power and that I like to do a few explosive exercises. My favorite is picking up a heavy bag and slamming it for reps. Another really good exercise for core strength is walking with a weight over your head. It's hard to do in most gyms and looks a little weird, but if you put 135 on a bar, put it over your head, and walk fifty feet you'll feel it.