From the Rik-parisistha:

"Srimati Radharani is always to be found with Sri Krishna, and Madhava is always to be seen with Radhika. One is never without the other's company."

From the Gopalottara-tapani:

"She is the embodiment of maha-bhava, or the highest transcendental ecstasy, and none of the other gopis, or cowherd damsels of Vrindavan, possesses Her excellent qualities. Radharani is therefore known as the most famous Gandharva."

In the Padma Purana Narada Muni describes Srimati Radharani:

"As much as Srimati Radhika is dear to Lord Krishna, Her pond Radha-kunda is as beloved to Him. Among all the gopis Srimati Radharani is most dear to Krishna.

In his Sri Prarthana-paddhati [Stavamala], Srila Rupa goswami prays:

"O Queen of Vrindavan, O Radharani, Your complexion is like molton gold, Your doe-like eyes are captivatingly restless, a million full and brilliant moons wane before Your lustrous countenance, and a blue sari, having stolen the hue of a fresh rain-laden cloud, has enwrapped Your exquisite form. O Radha, You are the crest-jewel of all the dallying damsels of Vrindavan, fragrant and pristine like a budding jasmine flower. Your sublime form is adorned with priceless jewelry , and you are the best of all the charming and intelligent gopis. You are decorated with all wonderful excellences and surrounded by eight dedicated and beloved cowherd girls known as the asta-sakhis.

"The ambrosia of Your beautiful lips, red as the bimba fruit, is life-giving syrup to Krishna. O Radha, I am rolling on the banks of the Yamuna, my poor heart filled with anticipation, praying to You with all humility. I am guilty of being an offender, a rascal, a useless wretch-yet I beg You to kindly engage me in even the smallest service to Your lotus feet. O most merciful Lady, it will not become You to ignore this most distressed soul, for Your heart is always overflowing with compassion and love."

Raghunatha dasa Goswami writes in the Vilap-kusum-anjali

"O Supreme Ladyship, Queen of my heart, Radha! Laksmi-devi the goddess of fortune does not possess even a drop of the beauty that exudes from Your exquisite toenails, therefore if You do not grant me the proper vision to perceive Your transcendental pastimes, then what use do I have for this life, which is burning in the fire of excruciating sorrow?

"O merciful Lady! Indeed, lately I am floating in a nectarean ocean of hope, and passing time in hardship and pain, but if You do not shower mercy upon me then this life, living in Vrindavan, and even Lord Krishna are all meaningless to me."

"The Queen of Vraja is my mistress. O Lady mistress, O Radha, I am Your maidservant, but the flames of intense separation are incinerating my heart and I grow feeble from profuse crying. Finding no other means, I am therefore sitting somewhere in Govardhan and composing these verses in deep lamentation.

"O dallying damsel of Vraja, Sri Radhika, I am sucked into an ocean of grief and my condition is so miserable! Kindly send me Your mercy in the form of an infallible boat and save me from this whirlpool. Please give me sanctuary at Your lotus feet."

"O Radha-kunda, pond of sublime joy, my mistress Srimati Radhika is always absorbed in divine amorous pastimes with her beloved paramour Sri Krishna on your banks, and you have endeared yourself to this Divine Couple more than anything else They cherish. Please, therefore, be merciful upon me and allow me but a moments vision of the object of my greatest adoration, Srimati Radharani.

From Brahma-Samhita

"I worship Govinda, the primeval Lord, who resides in His own realm, Goloka, with Radha, who resembles His own spiritual figure and who embodies the ecstatic potency [hladini]. Their companions are Her confidantes, who embody extensions of Her bodily form and who are imbued and permeated with ever-blissful spiritual rasa." B.S.  5.37

Sri Chaitanya Charitamrita  Adi Lila 1.6

Desiring to understand the glory of Radharani's love, the wonderful qualities in Him that She alone relishes through Her love, and the happiness She feels when She relishes the sweetness of His love, the Supreme Lord Hari, richly endowed with Her emotions, appears from the womb of Srimati Sachi Devi, as the moon appears from the ocean.

The essence of the hladini potency is love of God, the essence of love of God is emotion [bhava], and the ultimate development of emotion is mahabhava. Sri Radha Thakurani is the embodiment of mahabhava. She is the repository of all good qualities and the crest jewel among all the lovely consorts of Lord Krishna.

Of these two gopis [Radharani and Candravali], Srimati Radharani is superior in all respects. She is the embodiment of mahabhava, and She surpasses all in good qualities.  (Ujjvala-nilamani 2)

Her mind, senses and body are steeped in love for Krishna. She is Krishna's own energy, and She helps Him in His pastimes.[Adi 4.68-71]

O Partha, in all the three planetary systems, this earth is especially fortunate, for on earth is the town of Vrindavan. And there the gopis are especially glorious because among them is My Srimati Radharani.         (Adi Purana-Krishna to Arjuna)

Lord Caitanya appeared with the sentiment of Sri Radha. He preached the dharma of this age-the chanting of the holy name and pure love of God. In the mood of Srimati Radharani, He also fulfilled His own desires. This is the principal reason for His appearance.
C.C. Adi 4.238-272