What is the criteria for "underrated" in this case? Poultergeist was a hugely successful and well-remembered film that spawned many sequels, parodies, and even a video game! In fact, I could be wrong here (and somebody can look this up on wikipedia and prove me right or wrong), but I'd say it was easily one of the most successful horror films ever made.. not just fianancially but in its legacy/inspiration to others as well.
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