Atticus has many strengths in the book.
First, he is an amazing father to his children. He treats them with respect and he teaches them important lesson for life. For example, he wanted Jem to spend time with Mrs. Dubose to learn an important lesson on courage. Here is what Atticus says:
I wanted you to see something about her—I wanted you to see what real courage is, instead of getting the idea that courage is a man with a gun in his hand. It’s when you know you’re licked before you begin but you begin anyway and you see it through no matter what. You rarely win, but sometimes you do. Mrs. Dubose won, all ninety-eight pounds of her. According to her views, she died beholden to nothing and nobody. She was the bravest person I ever knew.”
Most importantly, Atticus teaches his children by modeling honorable actions through his own life.
Second, Atticus is a man of courage. He will do what is right, even if it is unpopular or even if he will lose in the process. The obvious example is his defense of Tom Robinson not only at court, but also in the face of a mob.
As for weaknesses, there is only one that I can think of. At the end of the novel, he failed to see, at least initially, that it was important to protect Boo Radley from Maycomb. In other words, Atticus was too bent on keeping the letter of the law that he forgot that the spirit of the law was more important.