I use magnesium orotate, as it is most easily assimilated by the body. It's never given me stomach problems no matter how much I take. Can't say that about mag citrate or others. And this is important as mag isn't easily assimilated in general and takes a long time to saturate the tissues.
However, it's not easy to find and a little more expensive than the others. I buy mine online at vitaminshoppe.com (brand "nutrient carriers/advanced research). But I have had many heart issues helped immensely by it. For me, it's worth it's weight in gold.