For me, it has to be “Old Boy” which is originally a Korean movie but the US version sucks so much that it remains incomparable. Another contender is “The Ring” again a Japanese movie (the ending differs between both versions, as the original one refers to the protagonist’s father sacrificing himself but the remake states that the cursed tape is being sent to someone else, passing the omen towards another stranger).
I don’t know if it’s the worst, but Speak No Evil (2022) has to be up there. It’s a bleak Danish-Dutch horror movie about the dangers of being a people-pleaser and ignoring red flags out of politeness. The 2024 American remake turned it into a pretty typical Hollywood horror movie with a happy ending, which completely undermines the whole point of the original.