CC Madhya 14.160

gopī-gaṇa-madhye śreṣṭhā rādhā-ṭhākurāṇī
nirmala-ujjvala-rasa-prema-ratna-khani
Word for word: 
gopī-gaṇa-madhye — of all the gopīs; śreṣṭhā — the chief; rādhā-ṭhākurāṇī — Śrīmatī Rādhārāṇī; nirmala — purified; ujjvala — brilliant; rasa — in mellows; prema — of ecstatic love; ratna-khani — the jewel mine.
Translation: 
“Of all the gopīs, Śrīmatī Rādhārāṇī is the chief. She is a jewel mine of ecstatic love and the source of all purified transcendental conjugal mellows.