I tend to think that anyone with neurological issues should take Methylcobalamin B12, as this cannot hurt--it has no known toxicity level--and is helpfully neuroprotective.

Magnesium tends to be my second choice overall--as Mrs. D points out, deficiency in this mineral is common, and again, there aren't too many side effects (except for loosening of the stools, which many need anyway).

After that, though, it's a much more individual question. In general, people with blood glucose regulation problems might benefit from lipoic acid and/or inositol and/or biotin; people with mitochondrial problems might want to try carnitine and/or CQ10. But one size doesn't fit all.

The sad part about this is that there are really very, very few people with the biochemical or nutritional background to make good professional recommendations. Certainly few practicing physicians are expert in these areas, and even many nutritionists are not familiar with the area. So, for a lot of us, it's a long period of trial and error.