I think the most popular is PHP, but *my* preference is ASP.NET, because it's much more structured, compiled and it's easier to create maintainable, testable apps with.

With Microsoft's new free tools, it's getting easier all the time:www.asp.net

In my opinion there doesn't exist a "best" language, it is all dependant on the reason and nature of your project. I normally use PHP because it's quick, easy to work with, and almost every server uses it in one way or another. But this doesn't mean that all of my projects rely on PHP...again, it depends on the project at hand.