beda jauh mas, yang baik sekali, (he/she/it/whatever) which or who, is really good/kind/fine.(not a superlative) terbaik, the best/kindest/finest.... for which you can also say 'paling'.., paling baik.(both superlatives)

yang baik sekali : the very kind. (attitude) ex. Saya memiliki nenek yang baik sekali. terbaik : the best. (quality, effort) ex. Saya mempunyai nilai terbaik.

dan yang terbaik pun belum tentu baik sekali..

menurut saya, jika YANG BAIK SEKALI itu belum tentu TERBAIK.

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