A free-form radial map is usually the most effective for casual brainstorming. Place the topic in the center, then rapidly add branches for ideas without worrying about structure. The goal is speed and idea capture rather than perfect organization.
Key tips:
After the initial burst, switch to refinement: cluster related nodes, promote important ideas to top-level branches, and prune duplicates. If you plan to convert brainstorms into action items, add tags or labels (e.g., "priority", "timeline") and extract tasks into a checklist or project board.