FreeGameDev

It depends on what you want to edit

There is no best software for open source games. Just like there is no best way to write a novel and no best design for a house. If you limit it to part of open source game development like writing code or creating 2d graphics, there are good software for doing that but nothing is the best. About the only way you can find the "best" is if you limit it to a vary narrow field. Like many the godot editor is the best for making game using the godot engine. (That just an assumption that there is an editor and no other way of editing it.)

Best software for what? Editing what?

That's like saying, what's the best paint for England? What do you mean, English houses? The outside of the houses? The inside? Or cars? Or a canvas? I mean, there's just so much breadth to the question that it's not even useful.