What’s the best product management/business development requirements gathering tool?

In my experience, the best way to track product/business management requirements has been to create a process that is specific to your business or department.

That process can be a simple one like:

1. Writing notes on a piece of paper
2. Whiteboarding requirements
3. Entering requirements into an email chain and replying to it whenever another idea comes up
4. Try out a Google Doc (docs.google.com)
5. Using spreadsheet tool
6. Using a text editor
7. Etc.

Or, you can work on a more complex process that allows collaboration and tracking:

1. Kanban Board
   1. Trello
   2. http://Asana.com
   3. http://Smartsheets.com
2. Flow Charts
   1. Flowchart Maker & Online Diagram Software
   2. Online Diagram Software & Visual Solution | Lucidchart
   3. Visio
   4. Flowchart Maker – Easy Online Diagramming
   5. Create Flowcharts, Floor Plans, and Other Diagrams on Any Device

When I work with companies that are looking for ways to improve upon their current process, I normally develop something custom tailored for their team.

One of the latest clients developed interactive exhibits and I worked with them on a process where they would utilize a combination of a Kanban board, Bi-weekly Brainstorming Meetings, and Requirments Analysis Meetings to build proper requirements for their needs.

Your situation is likely different, so try a process and tweak it until it works for you and your team. Sometimes it will require a bit of team training to get stakeholders on board.