I know what 'fly box' means to me-- might not be the same for you......
What are you wanting the box to do?
Does it need to be packable-- so as to fit in your vest pocket? Or do you carry a pack and want all of your flies organized into one (bigger) box?
Does it have to be waterproof? (Of course in a perfect world.....)
Capacity requirements? (again, in a perfect world......)
What do you want to spend?
Never used it, but based on features vs. price, I'd probably opt for something along the lines of the Flambeau blue ribbon fly box. --- removable leafs for ex. storage, foam hook keepers, water proof (ish)--- the only thing that it lacks is 'see through-ness'
. reasonably priced in the $20. range.
I can tell you what not to get.... I use old plastic tab closure, with plastic channel type keepers. One wet, one dry. They work. But they ain't slick and cool.
On the plus side, they are cheap.