This is very debatable, but my personal opinnion is the Great Battle in the Silmarillion. It is where the Valar themselves come and capture Morgoth and destroy his armies. Wihtout this happening,the entire history of Middle Earth would be extremely different since Morgoth would have drastically changed things.
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He came unto the timeless halls where shining fall the countless years and endless reigns the Elder King in Ilmarin on Mountain sheer
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