Which of the original 8 weight divisions do you know the least about? They are: Flyweight, Bantamweight, Featherweight, Lightweight, Welterweight, Middleweight, Light Heavyweight and Heavyweight. I'll say Featerweight.
Not to sound too big headed, but I'm decently versed in all of them from the top down, but I'd say when it comes to knowing less and less about guys who aren't great/more known, it's between 126/112/118.