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Dawn of the Dead is maybe slightly overrated. I would absolutely put it in any top 10 list, but I just tend to be more of a NOTLD fan. Shaun of the Dead is also a bit overrated IMO. It's hilarious, and one of the best horror films of the decade, but not one of the best of all-time like so many people say it is.
I would NEVER call The Exorcist overrated. The scariest film of all-time? Historically, maybe, but overall it didn't scare me too much. But nevertheless it's one of the best of the '70s. It is a true classic.