I think that the most memorable and effective scene was when Napoleon called in his nine dogs to attack Snowball during their last debate together about the windmill. This is a key turning point in the book, because this scene shows Napoleon's first move towards his new government of the farm, which is communism. This action eradicates Snowball from his position of leadership, and gives Napoleon sole leadership of the farm. Snowball is then shown as a traitor, a liar, and a thief. He is blamed when things go wrong. The reason this part is effective and memorable is because it's a key point in the book, and the rest of the book is shaped by it. The animals still follow Napoleon because he says that Mr. Jones will come back if they don't listen to everything he says. The animals hate Mr. Jones, so they will do anything so he doesn't come back to rule over them.
Napoleon tells the animals that it was recently discovered that Snowball was working for Mr. Jones. Naturally, some animals don't believe him, and they brought up points such as how he fought bravely in the Battle of Cowshed. He tells them that Snowball has been working with Mr. Jones since the beginning. This is very effective because none of the animals can really question Snowball's loyalty without also being deemed traitors, and killed. Napoleon also says that he believes that all animals are equal, but then establishes that only pigs can be in charge of the farm. That statement is very ironic, because it says that every animal is equal, but on the other hand, some animals are better than others. He even goes on to say that all animals are equal, but some animals are more equal than others.
Napoleon uses the dogs that were taken from their mothers at birth to attack Snowball, uses them as his personal defense, and uses them to force his orders on everyone. This transformation of the innocent puppies into killing machines shows that anyone can take what is good, and turn it into evil to do malicious deeds.