This is the 398th most frequent Persian word.
بهترین
Best.
In this sentence, 'بهترین' is used as a superlative adjective to indicate the highest degree of quality in describing the noun 'friend'.
او بهترین دوست من است.
She is my best friend.
Here, 'بهترین' identifies the superiority of one specific book among others read by the speaker.
این کتاب بهترین کتابی است که خواندهام.
This book is the best book I have ever read.
In this context, 'بهترین' is used to denote the optimal quality or preference among various choices.