I’m not sure what you’re asking, I don’t know what you mean by “top category”.
There is “top” as in visual position and there is “Top” as in the topic list page of “most bestest” topics as determined by an algorithm.
If you are referring to the Top topic list page, AFAIK, there is currently no way to game the algorithm (unless maybe a plugin could do it). You could however make a topic a Banner and it would display at the top “visually”. If you did not want the Banner to display on pages other than the Top page, I believe that should be easy enough to do with some custom CSS