As of January 2012, Dickens' A Tale of Two Cities was top of the list not only of his own books, but of the best-selling books of all time. This may be a surprise, as in recent years other books like Great Expectations and David Copperfield have had more media attention due to TV, film and stage adaptations.
However, A Tale of Two Cities has been popular for a long time. Its main subject, the French Revolution, was more topical in Dickens' day than it is today - even in the mid 19th century, people were still arguing about the pros and cons of the revolution, much as people argued about the 1917 Russian revolution for many decades after it happened.
As an international story it may have a global appeal that other Dickens novels don't have (apart from Martin Chuzzlewit, set partly in the US, most of his books are very British with a bias towards London). It is also quite short, which never hurts!