"Best at" is a transformed form of "good at." Good-->Better-->Best So to be the best at something means that you are not merely good at it, you are the best (no one is better than you). In the example you provided, to find the object of the preposition, just rearrange the words into a more tradition subject-verb-object sentence structure, so-->"Boxers are best at which part of English?" But we need to put the question word "which" at the start of the question, resulting in the original (correct) word order which you wrote. P.s, Some people say that the preposition should be moved to the beginning of questions like this, so it is clear what its object is, so "At which part of English are boxers best?" But this is a rather formal way of speaking, most people would say it the way you wrote it.