“Qui est un bon chien ?” Is the exact translation.

In reality we would probably say “C’est qui le bon chien (or Le bon toutou) ?” with a ‘aww’ voice.

Not much of the difference yeah, but the “C’est” at the beginning is important, it makes it sound more familiar and childish.

Tu es un bon chien? Oui, tu es un bon chien!