Orthopedic and biomechanical experts have been telling us for years that exercising the transversus abdominis (TA) is crucial to the support of the spine in exercise. Now that may have all changed. Read my article on the deep abdominals for more information. In light of this, I won't spend much more time on the TA except to say that utilizing a suite of abdominal muscle exercises, such as those below, in conjunction with abdominal bracing should provide sufficient work for the TA. (See Grenier and McGill in the Sources.)
In 2001, the American Council on Exercise (ACE) commissioned a study by Peter Frances at the Biomechanics Lab at San Diego State University in which they studied 30 men and women aged 20-45 with a range of fitness and weight training expertise. The researchers used electromyography (EMG) equipment to monitor muscle activity as they exercised.
Here are the top six exercises for the rectus abdominis and the obliques according to muscle activation scores. The relative score is to the right.