Why Gopis alone were the best examples for Perfect Krishna Consciousness?

Because they were always involved in the thoughts of Krishna all the time spontaneously!

SRILA PRABHUPADA SAYS IN SRIMAD BHAGAVATHAM CANTO-10:

Another gopi said that everything appeared wonderful when Krsna and Balarama traveled in the forest of Vrndavana playing Their flutes and making intimate friendship with all kinds of moving and nonmoving living creatures. When Krsna and Balarama played on Their transcendental flutes, the moving creatures became stunned and stopped their activities, and the nonmoving living creatures, like trees and plants, began to shiver with ecstasy. 

Krsna and Balarama carried binding ropes on Their shoulders and in Their hands, just like ordinary cowherd boys. While milking the cows, the boys bound the hind legs with a small rope. This rope almost always hung from the shoulders of the boys, and it was not absent from the shoulders of Krsna and Balarama. 

In spite of Their being the Supreme Personality of Godhead, They played exactly like cowherd boys, and therefore everything became wonderful and attractive. While Krsna was engaged in tending the cows in the forest of Vrndavana or on Govardhana Hill, the gopis in the village were always absorbed in thinking of Him and discussing His different pastimes. This is the perfect example of Krsna consciousness: to somehow or other remain always engrossed in thoughts of Krsna. 

The vivid example is always present in the behavior of the gopis; therefore Lord Caitanya declared that no one can worship the Supreme Lord by any method which is better than the method of the gopis. The gopis were not born in very high brahmana or ksatriya families; they were born in the families of vaisyas, and not in big mercantile communities but in the families of cowherd men. They were not very well educated, although they heard all sorts of knowledge from the brahmanas, the authorities of Vedic knowledge. The gopis' only purpose was to remain always absorbed in thoughts of Krsna.