Boxer is arguably the best character in George Orwell's Animal Farm because he possesses all the attributes most of us admire. He is loyal, kind-hearted, and hard-working, always willing to do more if it will benefit the common good. Of course this novella is an allegory for the people and events of a specific time in Russian history, so Boxer is more than just a good cart horse; he represents the common, working man.
Without Boxer, nothing on the farm would get accomplished as easily, and some things, like the windmill, would not be done at all if it were not for Boxer. He is not the most intelligent animal on the farm, but he is willing to question authority when he feels things have gone too far. In general, though, he tends to believe that if he works hard his efforts will eventually be recognized and he will be taken care of in his old age by the leaders who need him to work.
Of course we know that does not happen and, when he has outlived his usefulness on the farm, he is sold to the knacker for some whiskey for the pigs.
Boxer's mottoes are inspirational to the others:
"If Comrade Napoleon says it, it must be right."
"I will work harder."
"Napoleon is always right."
He is trusting (perhaps too trusting) of the pigs (the government), and when he has the chance to change things he does not even realize it. He has one of his hooves on Napoleon's dogs, unwilling to believe the creature could have been about to attack him; if he had taken action then, everything would have changed for the animals under the pigs' oppressive rule. Instead he consults Napoleon and lets the dog go free.
Boxer represents the people who do what they should for themselves and others but is mistreated by the very government that uses his strength and work to become an oppressive entity.