Choosing the best forum software for a community with over a million members requires careful consideration of various factors, including performance, scalability, features, and customization options. Here are some forum software options that are known for their ability to handle large communities:

1. Discourse: Discourse is a popular open-source forum platform designed for modern communities. It’s highly customizable, offers a clean and intuitive user interface, and is built with scalability in mind. It can handle large communities effectively and has a strong developer community.
2. vBulletin: vBulletin is a well-established commercial forum software that has been around for many years. It offers robust features, scalability, and a strong user management system. It’s known for handling large forums effectively.
3. XenForo: XenForo is another commercial forum software with a reputation for performance and scalability. It provides a modern and user-friendly interface along with features suitable for large communities.
4. phpBB: phpBB is an open-source forum software that has been around for a long time. While it may require more customization and additional plugins to handle extremely large communities, it’s a solid and free option with an active community of users and developers.
5. Invision Community (formerly IP.Board): Invision Community is a commercial forum software that offers a comprehensive suite of community-building tools. It is known for its scalability, user-friendly interface, and a variety of features that can cater to large communities.
6. MyBB: MyBB is an open-source forum software that’s free and has a strong community. It may require some customization and optimization for very large communities, but it’s a solid choice for those on a budget.
7. NodeBB: NodeBB is an open-source forum software built using Node.js. It’s known for its real-time functionality and can handle large communities effectively with the right server setup.
8. Flarum: Flarum is a modern open-source forum software built using PHP and Laravel. While it’s still in active development, it shows promise and can be a good choice for those looking for a lightweight and modern forum platform.

When selecting forum software for a community of over a million members, consider factors such as hosting requirements, customization options, security features, and the specific needs of your community. It’s also essential to invest in powerful hosting infrastructure, content delivery networks (CDNs), and optimization to ensure the best performance and user experience for your large community. Additionally, thorough testing and performance monitoring will be crucial to maintaining a smooth user experience as your community grows.