This is the 9714th most frequent Turkish word.
arasındaydı
"Was between" or "was among."
The word 'arasındaydı' in this context indicates that the subject was considered among a certain group ('en başarılı olan').
Öğrenciler arasında en başarılı olan o arasındaydı.
Among the students, he was one of the most successful.
Here, 'arasındaydı' is used to indicate that the object was physically located within a group or collection ('among the books').
Kitapların arasındaydı ve dikkatini çekmiyordu.
It was among the books and was not drawing attention.
In this usage, 'arasındaydı' shows inclusion within a list or group ('among the participants').