Which motif or symbol is the most prevalent or effective in the novel? What makes this motif/symbol so effective?

Expectations vs reality is a recurring motif in the narrative. Pip drew this mental picture in his head that London would be a grand and glittering place. It was in fact dirtier than the countryside that Pip came from. Pip assumed that Miss Havisham is his benefactor. When in actuality, a lowly criminal who happened to be Pip's convict, is his benefactor. Pip also expected that Estella was meant for him. But now he knows that Estella had not been part of his Great Expectations and that the probability of that happening is non-existent.