His two films were the most unmemorable in the series. They lacked iconography and failed to become cultural icons that many of the other films in the series did.
Neither have a memorable villain. In The Living Daylights, the archetypal bad guy isn’t introduced until the film is half-way through and their plans for world domination are vague and ill-prepared, hardly giving Dalton’s Bond character a worthy nemesis. Similarly, the South American drug baron that constitutes the baddie in License to Kill is equally unmemorable.
With such unmemorable films, and never having himself tested against a worthy opponent, it would be improper to crown Dalton as the best Bond of all.