General Brazilian
ser desinteressante / ser chato
Both expressions are commonly used; 'chato' is more colloquial
Rio de Janeiro
ser chato / ser entediante
'Chato' is very common in everyday speech
São Paulo
ser desinteressante / ser monótono
Formal contexts prefer 'desinteressante'
Portugal
ser desinteressante / ser aborrecido
'Aborrecido' is more frequently used in Portuguese Portuguese