The scene where Boxer dies is a very important part of the story because they lost one of the most valued animals on the farm and it led to seeing the pigs do something unexpected.
When the pigs started walking on hind legs it completely changed they way they saw things. They even started saying 4 legs good, 2 legs better.
I think one of the most important scenes in Animal Farm is when the pigs start to drink because the animals start to notice the rules seem weird. Over time the forget more and more but at that point they felt something was off. Not to mention their drinking would later get out of hand.
The battle of the Cowshed was one of the most important parts because it was the biggest part of the story. A war between man, animals, and farms.
The rebellion is easily one of the most important parts of the story because this is how they gained their freedom.
When the animals look and see no difference between animals and man is a huge part of the story. It may be the ending but the pigs had changed so much to the point they were basically considered a man.