Who has the best cupcakes in Rockville? We want your opinion. This is the debut of a new feature called "Rockville Rivals," where we ask readers to visit our "Places" section and weigh in on what they like best. You don't have to be a professional critic, just jot down a few thoughts on what you like (or don't like) about the selected places. Here is how to play: 1 Create a free Patch login, if you don't already have one, by clicking on “Log In” at the upper right of this page. 2. Click the links below to visit the listings in our Places directory. Today’s matchup: In this corner, from King…