1. someone or something in a position of power and authority
he/she is top dog at the company
The 'top dog' is a person or organization with more power, authority, or influence than its rivals. Usually, the position of 'top dog' is established after intense competition or success, which is why the expression is much more common in a professional context where competitive hierarchies exist. Similar expressions include 'head honcho,' 'the big cheese,' and 'kingpin.' Its antonym is 'underdog.' Although this expression is more common in professional contexts it is an informal expression.