To be precise, there is no as such best content filter available for WordPress as all of them are different from each other and have different features available with them. Attainable at monthly fixed prices, some of them are sophisticated and advanced ones that allow additional spam filters and false positive taboo detection. Few of them are available at free of cost, though lacked with extra features including built-in profanity lists.
Check out more details at http://science.opposingviews.com/content-filters-wordpress-23162.html.