It's an unfair question since no one can guess individual needs, considering there are so many variables. What do you have already? What do you like taking pictures of? What is your budget? What is the intended end use of your images? What kind of camera do you have? What is... well, you get the idea.
While the above question may be a common one the core problem is the lack of knowledge about lenses in general. I will try to cover some of the more common answers here, just keep in mind that it is by no means complete nor comprehensive.