I would agree that Boo is a hero, but mu reasoning is a little different and might be worth exploring. There is no doubt that his family isolated Boo. This isolation and their treatment of him, might have been enough to cause Boo to give up on life, but he didn't. He might have given up on emotional connections to other human beings, but he didn't. He continued to care and care deeply. This makes him a hero.
Along with others named for bravery, Miss Maudie also displays bravery as she retorts constantly to the hypocrites of the town despite what they may think.
But, for heroism in the old sense of the word--not that of the 21st century-- Boo Radley is truly the most heroic for the reason already stated in Post #3. With no consideration of the dangers to himself, Boo comes to the rescue of his friends simply because they need help and he loves them.
I think Boo is the most heroic over the course of the whole book. He gets himself totally outside of his comfort zone to help the kids at the end of the book, but also when he does things like putting the blanket over Scout at the fire, for example.
I also think that Calpurnia is heroic at times. For example, she stands up to Lula to defend Jem and Scout when Lula says they shouldn't be at the church. That's brave in itself -- standing up against others of your race in such a racially divided place.
You have several characters who show traits of heroism in To Kill a Mockingbird. Mrs. Dubose is described by Atticus as " 'the bravest person I ever knew.' " She shows a different kind of courage in her decision to rid herself of her morphine addiction before dying. Boo Radley shows a more obvious type of heroism in the final chapters when he comes to the rescue of Jem and Scout. Atticus Finch certainly shows his own brand of bravery when he chooses to defend Tom Robinson, knowing that he and his family will suffer the wrath of many of Maycomb's citizens. Jem Finch shows a heroic streak when he defends Scout against Bob Ewell and when he stands by Atticus in the face of the lynch mob. Take your pick. I would probably choose Boo because he is such an interesting character.