Magnesium biglycinate and Magnesium l-threonate generally have the best effect with the least laxative side-effects (unless you want that); however, l-threonate

Magnesium biglycinate and Magnesium l-threonate generally have the best effect with the least laxative side-effects (unless you want that); however, l-threonate is quite expensive because of patents, and l-threonate can have (potentially desirable, however potentially dependence building) effects of its own. One form I haven't tried or researched yet is sucrosomial magnesium, which may have superior bio-availability to both biglycinate and l-threonate with relatively reasonable pricing characteristics. I personally stopped taking magnesium a little while back, as it seems any bioavailable form pretty reliably causes insomnia for me. YMMV with magnesium, but you gotta make sure you buy it from a reliable source in a good form if you want to give it a proper chance.