There are plenty of guides mentioning regions to check. So you may as well check them all.
As we know you have to step foot in the region, not necessarily have them uncovered by sailing past, etc. So, my tip is:
- View the your map with All Icons Enabled.
- In each region check to see if you have completed a task, like a sync point, a camp, a tomb, etc. This will be shown with a tick over the icon.
- If you have a sync point or a ticked task then you know you've set foot in the region.
- If not, set a marker on a ? or area in the region, then fast travel nearby and travel to the marker in the region.
I was doing this and found my unvisited region, even though I had scoured the map for ages.