the.com/top dog

means The person or group with the most power, status, or success in a given situation.

from From 19th-century dog-pit slang, when fighting dogs were literally judged top or bottom — the dog pinned beneath in a fight was the 'underdog,' and the one standing over it was the 'top dog.' The phrase jumped from the pit to general use by the mid-1800s, keeping its winner-takes-the-high-ground logic intact.