While this may look like a technical question, it is actually an operational question. Rather than ask you about a specific element of web development and design, the interviewer is asking you about a process you use to develop a website. Their objective in asking this question is to determine if you have done work similar to the job for which they are hiring you. Based on your pre-interview research, you should describe a project similar to the type of work you will be doing with this organization.
"The most challenging website I have developed from start to finish is very similar to the main one your company uses. This involved online transactions and forms that users fill out to apply for the services the organization I was working for provided. The challenge was to make the website adaptive across multiple browsers and devices. I achieved this using a standard format and several different plugins available for that format."
"While I have worked on many different types of websites, the most challenging one involved a site that would be visited by users across the globe. Not only did I have to make it adaptive across multiple platforms, browsers, and devices, but it also had to be able to be translated into multiple languages. I was able to accomplish this using a set of tools specific to this task and a format that would accommodate all of these requirements."