Best Brainstorming in public tool: Your pick?

If you had to pick one tool available now to allow brainstorming on multiple topics that is *open to anyone* to contribute (think unlimited team members) what would be your choice? I'd like to invite an unlimited number of people to mind map ideas and comment in the most open way possible. What brainstorming tools have you used that you think might work in this context? I'm trying to get away from the traditional forum and go for a more graphical, chaotic and explorative interface that's a bit like an ever-expanding galaxy 😜 Love to get your thoughts and recommendations....