The ab wheel is OK, especially for the lower abdominals and hip flexors, but the best overall ab exercise is a weighted crunch on a Swiss Ball, IMO.
A Swiss Ball, for those not in the know, is a thick-skinned inflatable ball maybe 40" in diameter. For this particular exercise, you anchor your feet and then drape your back over the ball. This gives your abs a full stretch; floor crunches in comparison only work 1/2 of the muscles' range. Then you do a slow crunch, bringing your ribcage closer to your pelvis, and stopping the upward motion when you can no longer shorten that distance. When you get strong enough, hold a weight to your chest. As with all muscle groups, progressive resistance is the key to continued improvement and muscle growth.
An even more sophisticated version is to do the positive (raising) portion of the ab crunch holding a weight to your chest, then lifting the weight above your head for the negative portion. This selectively overloads the eccentric portion, where muscles are much stronger.
Swiss Balls are pretty cheap. I picked mine up for $15 at Target (not labeled as a Swiss Ball; it may even have been from Denise Austin...).
For lower abdominals, I favor leg raises on a slantboard, hands placed just beneath the butt, with a pelvic tilt and pause when the legs are perpendicular to the floor. Most people will find this challenging enough without a slantboard, so you might give it a try.