فِي أَحْسَنِ تَقْوِيْمٍ

فِي أَحْسَنِ تَقْوِيْمٍ is a phrase (شِبْهُ الجُمْلَةِ) consisting of a preposition followed by a possessive nominal compound

PhraseParticle/Noun/NounNounNounParticle
شِبْهُ الجُمْلَةِ جَارٌ وَمَجْرُوْرٌ وَهُوَ مُضَافٌ وَ مُضَافٌ إِلَيْهِمُضَافٌ إِلَيْهِاسْمٌ مَجْرُوْرٌ وَهُوَ مَضَافٌحَرْفُ جَرٍّ
PhraseCompoundPossessive NounGenitive noun/Noun possessedParticle of pulling down
  
in the best of formفِي أَحْسَنِ تَقْوِيْمٍ=تَقْوِيْمٍ+أَحْسَنِ+فِي
 in the best of formformthe best ofin
Surah/Below is the only verse in the holy Quran containing the compoundفِي أَحْسَنِ تَقْوِيْمٍ
Verse
95-4Certainly, We created man in the best of formلَقَدْ خَلَقْنَا الْإِنْسَانَ فِي أَحْسَنِ تَقْوِيْمٍ