. . . And don't automatically say Slytherin just because he was a huge ponce. ;)
Seriously, though, using canon as a guide, what house would Gilderoy Lockhart have been Sorted into? He certainly had ambition, which Slytherin prized. It took a lot of nerve to pull off taking credit for other people's accomplishments for as many years as he did, and nerve and courage are Gryffindor traits. He worked hard at maintaining his façade for years; hard work is a Hufflepuff trait. While the accomplishments he claimed in his books were not his own, he did write a solid number of books (assuming he didn't have a ghost writer) and to effectively write for an audience takes intelligence, the main Ravenclaw trait.
We also know his magic wasn't very good. Perhaps this doesn't mean much, for Neville's magic wasn't strong and he was Sorted to Gryffindor. Anyhow . . .
Keeping this info -- and whatever else you know from canon -- in mind, which Hogwarts house would Gilderoy Lockhart have been Sorted to?