best-kept
/ˌbɛs(t)ˈkɛpt/adjective
1That is maintained in the best order or condition; most carefully cared for or looked after.
- In the United Kingdom now frequently in the names of competitions..
2Of a secret: that is known to very few people. Now used especially with reference to something good or desirable that is not generally known about.
Origin
Late 16th century; earliest use found in Vicary's Profitable Treatise Anatomy. From best + the past participle of keep, after well-kept.