Benign fasiculations for 2 years. Magnesium level 1.8, in range, but low. I want to try magnesium but what is best that won't cause diarrhea?

Magnesium glycinate. Magnesium oxide is the most commonly manufactured, due to its ease of manufacturing and low production cost--but it is not water soluble and can be poorly absorbed. Magnesium citrate pills have a high bioavailability, but the citric acid is a mild laxative. Magnesium glycinate is well-absorbed and reportedly the least likely to cause diarrhea. This one may be best option for correcting deficiency.
Glycinate or Taurate. Many with "normal" serum magnesium levels are still deficient so this is reasonable to try. I agree with Dr. Carr's fine answer recommending Mag. glycinate but another good option is Magnesium taurate. Epsom salt baths are also proven to raise magnesium levels. See for more on magnesium and health.