This is the 416th most frequent Swedish word.
vilken
"Vilken" means "which" in English.
'Vilken' is used as an interrogative to inquire about a specific choice among options (gender: common singular).
Vilken bok är din favorit?
Which book is your favorite?
Here 'vilken' functions as a relative pronoun connecting the main clause to the secondary clause providing additional information.
Känner du till filmen, vilken blev en stor succé?
Do you know the movie, which became a huge success?
'Vilken' expresses 'what a...' as an exclamatory form to describe something emphatically.