General Brazilian
ser entediante / ser chato
Both terms are commonly used; 'chato' is more colloquial
Rio de Janeiro
ser chato / ser maçante
'Chato' is preferred in informal speech
São Paulo
ser entediante / ser chato
Both are equally used in formal and informal contexts
Portugal
ser aborrecido / ser chato
'Aborrecido' is more formal; 'chato' is colloquial