Caitanya-caritāmṛta 4.193
ratna-gaṇa-madhye yaiche kaustubha-maṇi rasa-kāvya-madhye taiche ei śloka gaṇi
[ratna-gana-madhye yaiche kaustubha-mani rasa-kaavya-madhye taiche ei shloka gani]
Blended reading (Sanskrit + English inline)
ratna-gaṇa-madhye yaiche kaustubha-maṇi rasa-kāvya-madhye taiche ei śloka gaṇi
As the Kaustubha-maṇi is considered the most precious of valuable stones, this verse is similarly considered the best of poems dealing with the mellows of devotional service.
My note
Source: vedabase.io