أَحْسَنِ

A word in the Arabic language will either be 1) a verb or 2) a noun or 3) a particle – أَحْسَنِ is a noun

A noun may either be 1) Definite or indefinite 2) Masculine or feminine 3) Singular, dual or plural 4) Nominative, accusative or genitive

الأَحْسَنُ is the natural state of a definite noun and becomes الأَحْسَنِ due to: 1) a preposition that precedes it or 2) a possessed noun that precedes it or 3) it’s a noun of dependent declension a) نَعْتٌ / صِفَطٌ adjectives b) التَّوْكِيْدُ emphatic c) المَعْطُوْفٌ conjunction d) البَدَلُ synonym

الأَحْسَنِ loses it’s definite article, becoming أَحْسَنِ due to it being a possessed noun

Best

To be handsome, make good, seem good or beautiful, be excellent

Surah/Below are the 5 verses in the holy Quran containing the genitive nounأَحْسَنِ
Verse
and We wrote in the tablets of everything, an instruction and an explanation for everythingوَكَتَبْنَا لَهُ فِي الأَلْوَاحِ مِنْ كُلِّ شَيْءٍ مَّوْعِظَةً وَتَفْصِيْلًا لِّكُلِّ شَيْءٍ
“So, take it with firmness and command your people (so they may) take the best of itفَخُذْهَا بِقُوَّةٍ وَأْمُرْ قَوْمَكَ يَأْخُذُوا بِأَحْسَنِهَا
7-145I will show you the home of the defiantly disobedient”سَأُوْرِيْكُمْ دَارَ الفَاسِقِيْنَ
16-96
16-97
39-35
95-4Certainly, We created man in the best formلَقَدْ خَلَقْنَا الْإِنْسَانَ فِي أَحْسَنِ تَقْوِيْمٍ