The Now magnesium malate used to be a favorite but then they added calcium, which reduses its effect.

It was a big difference for me to get hold of a magnesium supplement without binders, fillers, calcium, coating, stearic acid etc.
Much more effective. I am doing fine on 2-400 mg of magnesium. This has for me been the single most difficult supplement to find out of.

I would up until bowel tolerance just to know where my limits was, same as vit C.
To maximize the uptake and utilization of magnesium; vit B6 are needed, so are boron.

AAC ?? o-o-o-h, I know the answer to that! AAC=amino acid chelate

Here is a good magnesium info, posted before;

"Unfortunately, magnesium deficiency is not easily detected, as serum levels do not reflect the levels of magnesium in tissues. This is the reason why it is so overlooked and ignored, both by doctors and by studies. And unfortunately, oral magnesium supplementation can be difficult because of absorption problems. Digestion and diet play a key role in absorption. People with fibromyalgia often have conditions like Irritable Bowel Syndrome, gluten intolerance, or other problems that might limit absorption. Excess amounts of certain substances, such as fructose, may interfere with magnesium absorption. Phosphate can bind to magnesium in the gut, creating magnesium phosphate, an insoluble salt that can't be utilized. Many forms of oral magnesium supplements are hard to assimilate. The most common, magnesium oxide and citrate, happen to be the worst to assimilate, which is why both have a strong laxative effect. If you suffer from that effect when you take magnesium, it is often not because you are taking too much, but because you are not assimilating it well. And it may take long term use of supplements before magnesium levels are raised in all the tissues, and for damaged cell functions to be restored"

Also note that B vitamins are necessary for proper utilization of magnesium. Some people with fibromyalgia might have B vitamin deficiencies, especially B12. One study, has shown that homocysteine levels are high in the cerebrospinal fluid, and this indicates low levels of B12 in the brain. Additionally, homocysteine causes a depletion of intracellular free magnesium:, and according to the following study, only a combination of B6, B12, and folate acid can stop this depletion of magnesium. Thus, some people with fibromyalgia might benefit from B vitamin supplementation. (However, certain B12 deficiencies require B12 shots, as some people are unable to orally absorb enough B12.)

In some people, the problem might not be enough magnesium, but instead might be not enough vitamin D, which has an effect on magnesium absorption. Up until recently, doctors only associated a vitamin D deficiency with weak bones, and they believed that a vitamin D deficiency in young people was rare in countries like the US. However, both of these beliefs have been proven false. Not only does it occur in young people, but sometimes a vitamin D deficiency can manifest itself mainly as pain. Initial symptoms in adults can often appear as vague musculoskeletal pains, muscle weakness, and fatigue, symptoms commonly found in fibromyalgia. Also, studies on the muscles of vitamin D deficient patients, show a reduction of ATP levels, similar to that of fibromyalgia patients.

http://web.mit.edu/london/www/magnesium.html

The product jbfun mentioned looks good. I would go for capsules with as little additives as possible. I believe Source Naturals has a good one to.
Malate, citrate or chloride. One other seldom mentioned is Magnesium ascorbate. It used to be a favorite of mine, but I cant get hold of it anymore. NOW has a great powdered one, tastes good and you get both vit c and magnesium in one go.