I'm guessing the best backend would be anything that does serverside rendering (SSR), like PHP. This means the HTML page is built on the server dynamically before it is sent to your web browser. Many people on the darkweb use javascript blockers so any sort of interactivity will likely be broken, so no React or whatever people use nowadays. PHP is a great choice because of how old it is and how easy it is to set up, though anything is fair game. I personally use Blazor (C#) for my website and it lets me create reusable components and easy integration with databases through EF Core