A superlative adjective is the result of comparing three or more nouns and indicates the one having the extreme or highest degree of that adjective. Persian superlative suffix is “-in”. There is only one way of forming superlative adjectives (compare with English “happiest” vs. “most beautiful”). Additionally, there are no irregular superlatives (compare with English “worst” and “best”).

Formation of Superlative Adjectives

The superlative form of an adjective is made by suffixing ین (-in) to its comparative form. In other words, by suffixing ترین (-tarin) to an adjective:

Adjective Comparative Superlative
بد
bad
bad
بدتر
badtar
worse
بدترین
badtarin
the worse
بزرگ
bozorg
big
بزرگتر
bozorgtar
bigger
بزرگترین
bozorgtarin
the biggest
به
beh
good
بهتر
behtar
better
بهترین
behtarin
the best
خوب
khub
good
خوبتر
khubtar
better
خوبترین
khubtarin
the best
زیبا
zibâ
beautiful
زیباتر
zibâtar
more beautiful
زیباترین
zibâtarin
the most beautiful
دوست‌داشتنی
dustdâshtani
lovable
دوست‌داشتنی‌تر
dustdâshtanitar
more lovable
دوست‌داشتنی‌ترین
dustdâshtanitarin
the most lovable

Both beh and khub mean “good” but the common way of saying “good” is khub. In contrast, “better” is normally expressed with behtar. Similarly, behtarin is normally used to mean “best”.

Adjective Comparative Superlative
به
beh
good
بهتر
behtar
better
بهترین
behtarin
the best
خوب
khub
good
خوبتر
khubtar
better
خوبترین
khubtarin
the best

Therefore, it is incorrect to say that خوب has the irregular superlative form بهترین.

Place of Superlative Adjectives

Superlative adjectives come before the noun they modify:

بهترین دوستم
behtarin dust-am
my best friend
این بهترین رستوران شهر است
In behtarin resturân-e shahr ast
This is the best restaurant of the city
زیباترین زنی است که می‌شناسم
Zibâtarin zan-i ast ke mishenâsam
She is the most beautiful woman I know