Magnesium glycinate is typically recommended, but any form that doesn't upset your stomach is probably good.

I currently use magnesium oxide in a tablet form (I use half a tablet, ~125mg). I have previously used magnesium glycinate but I don't remember what dose I used of that.

I'll add, the mag oxide always prevents leg cramps when I take a full tablet (250mg) when I am working outside all day (and staying hydrated).

ETA: In my experience magnesium citrate is the one mostly likely to cause diarrhea though they all can.

I use a powdered drink called 'Calm'. It's got a bit of a berry flavor to it. Start with a small amount though or you'll be shitting your pants. There are a lot of studies showing magnesium levels are far too low in people anyway so it's a good idea to supplement it.