In Animal Farm, who are happier, selfish or unselfish people?

Since Animal Farm focuses primarily on animals as proxies for people, I am going to assume you mean both animals and people when you ask about selfishness. Answering this question also means defining happiness. What makes us happy? Having material goods and power for ourselves or living with integrity and having strong relationships? I would say Orwell says both are necessary, but that he puts much more emphasis on integrity and relationships (unselfishness). Thus, the happiest characters are those who stay true to their natures and loyal to their friends. These are the unselfish animals, unselfish because they are willing to put their own safety and pleasure on the line for others. Snowball, who fights so bravely in the Battle of the Cowshed, would fall into this category, as would Boxer and Clover, who work endlessly and tirelessly for the community. Even Benjamin, who doesn't support the revolt and doesn't think life will get better, remains devoted and loyal to Boxer, coming to tend to him when he, Boxer, gets old, even after Benjamin has worked long days. These animals don't have much materially, and Orwell condemns that, but in the end they have, ironically, their "humanity" (or animalism), a quality of integrity that can't be corrupted and that cares for others. Orwell strongly sympathizes with these animals. These are the characters we love and care about and for whom our hearts break when they are betrayed.