Do you have a preferred brand?
Not really. I get stuff several different places based on what I happen to need the most. My one requirement for magnesium is to get high potency tablets with 200mg elemental magnesium per tablet. The choice of magnesium glycinate is because glycine is pretty useful as well, and it is one of the better absorbed varieties.
If it is of interest, Dr. Thomas Levy recently published "Magnesium: Reversing Disease", which is a great book on the subject. Funny story with him...he didn't see the need for the book and didn't want to write it, but was pushed by his publisher. After digging into the recent research on magnesium, he came to the realization that it was a major player in the cause of many diseases of age, which is calcium wandering away from where it should be and into many places it shouldn't. It was a major "aha" moment for him and completely reversed his view on the importance of magnesium.
The summary point is that magnesium is the natural calcium channel blocker and a major contributor to proper cell function. We tend to think old people need lots of calcium, but the truth is they probably have plenty, it's just in tissues it shouldn't be. Literally nothing good comes from that...causes Intracellular oxidative stress and progressive disease states...A Very Bad Thing. By restoring magnesium to optimal levels (along with Vit D and K), calcium gets restored to the places it should be and removed from the places it shouldn't, and dang near every disease of old age improves or even reverses. While almost everyone gets plenty of calcium, almost none of us get enough magnesium, and even when your doctor checks it, our serum level is often normal. This is because 99% of magnesium is in our cells, not our blood, and our bodies do an amazing job of rationing magnesium to critical things...like keeping your heart beating and your brain firing...but will take it away from less critical things to do so.
tldr...I could write a book on it, but Dr. Levy already did, so I'll shut up.