Here’s a quick answer. This is consistent across the board as far as I’ve seen. This below is from Dr. Hyman.
The most absorbable forms are magnesium citrate, glycinate taurate, or aspartate, although magnesium bound to Kreb cycle chelates (malate, succinate, fumarate) are also good. Avoid magnesium carbonate, sulfate, gluconate, and oxide. They are poorly absorbed (and the cheapest and most common forms found in supplements).
Side effects from too much magnesium include diarrhea, which can be avoided if you switch to magnesium glycinate.
Epsom salts (magnesium sulfate) is a good way to absorb and get much needed magnesium.
I guess if you bathe in magnesium sulfate it’s different… I guess. If you want to read, a lot, about Magnesium you can go to Dr. Groups site at globalhealingcenter.com
I still take the liquimins sea minerals to get the balance. You can look into that at www.traceminerals.com