If the question which subsector we think is actually least used, I don't quite see how one would go about figuring it out. A process of elimination doesn't get me very far. I believe Regina and District 268 are the ones with most adventures written for them, with Aramis and (I like to think) the Sword Worlds close behind. After that I'd be hard pressed to divide the remaining 12 into the middle six and the bottom six, let alone select one for the bottom spot. Perhaps something like this: Middle six (in no particular order): Five Sisters, Mora, Darrian, Lanth, Glisten, Vilis. Bottom six (again in no particular order): Cronor, Querion, Jewell, Lanth, Lunion, Trin's Veil.
If the question is which one we each think the least interesting... well, that's a toughie. I can think of interesting adventures for every one of them. I've either written adventures for or have notes for adventures set in every one except Querion and Cronor, and that's just sheer happenstance.
Hans