This dictionary is considered ----- a great help to students

consider作"认为"时,后面接一个宾语和一个动词不定式作宾语补足语,构成复合宾语结构
例如:①We always consider him to be a weak leader.我们一直认为他是个能力不强的领导.
　　②They consider Jim (to be) the cleverest boy in their class.他们认为吉姆是他们班最聪明的学生.
你问的句子为被动,所以为considered to be.