This is another behavioral question that is similar, but opposite of the previous one. Asking this question after the last one about your least favorite web site provides the interviewer with a clear idea of what you like and dislike. This enables them to determine if you are suited for the type of projects they will be assigning you. Again, use your pre-interview research to identify projects that align with those the employer will likely assign you to work on.
"My most successful website was for a project in which I was allowed a complete creative license. The customer described what they would like to achieve with the website and gave me the freedom to develop it anyway I chose. This released my creative abilities and resulted in a website that not only looked good but functioned well and achieved the customer's objectives."
"The most successful website though I worked on was one in which the objective was clear and well defined. It involved close collaboration with the client and other members of the development team. Since we were all on the same page, it was easy to create a web site with the exact elements and functionality the client was looking for. As a result, we were able to develop the website and about half the time it normally takes. The final product generated metrics that exceeded the client's expectations."