Helm or Waist.
If you go a decent helm, go waist, otherwise go helm. The slot you put it in really doesn’t matter, because the ilevel of legendary is equivalent to the ilevel of the epic gear you’ll be equipping. So just put it in whichever of the 2 slots you have the worst gear.
You can always craft a second one later on when you have spare resources, for the other slot, potentially even with different stats.