I'm a big fan of these binders from Bindertek. They have hard sleeves that allow them to stack nicely on top of each other. That way the pages aren't sagging from the weight. And you can slide each binder in and out easily without having to move the whole stack around. They're a bit pricey, but I think they're worth it. High quality binders. They come in various colors and sizes. I like the black ones.
Link to 10-packs:
https://www.bindertek.com/one-touch-...ne-pack-of-10/
Link to individual binders:
https://www.bindertek.com/binders/3-...assic-binders/