What breed of dog has won the most best in show titles?
The Cocker Spaniel has won the most BiS (Best in Show) titles. The Cocker Spaniel breed has won 8 times already while Welsh Terriers and Irish Setters have each won 4 times only.
The English Cocker Spaniel is a breed of gun dog. The English Cocker Spaniel is an active, good-natured, sporting dog  standing well up at the withers and compactly built. There are “field” or “working” cockers and “show” cockers. It is one of several varieties of spaniel and somewhat resembles its American cousin, the American Cocker Spaniel, although it is closer to the working-dog form of the Field Spaniel and the English Springer Spaniel.
Outside the US, the breed is usually known simply as the Cocker Spaniel, as is the American Cocker Spaniel within the US. The word cocker is commonly held to stem from their use to hunt woodcock.