Wherever possible, you should only write about a topic to which you feel you can do justice. The list of questions you've provided gives you quite a lot of scope, so that shouldn't be a problem. Personally, if I were writing the essay, I'd plump for number 3 because Atticus is such an important character in the story. As such, there's an awful lot you can write about his character.

In writing the essay, you wouldn't simply list Atticus's character traits; you'd illustrate them in depth using the countless examples scattered throughout the book. For instance, if you wanted to discuss his modesty you'd mention his reluctance to tell his children about his skill as a marksman. Likewise, if you were going to highlight his empathy, you'd show how he always tries to put himself in other people's shoes. A good example of this would be the unpleasant incident with Bob Ewell, when he spits right in Atticus's face and vows to get him even if it takes him the rest of his life. Despite being on the receiving end of Bob's abuse, Atticus understands exactly where he's coming from; he'd completely undermined Bob's credibility on the witness stand during the trial of Tom Robinson and so Bob felt humiliated and desperately wanted revenge.

First, I would suggest you choose the topic with which you feel most comfortable and which you best understand. I would probably choose the "minor characters" topic, though that may actually be the least popular choice among others. I find many of the minor characters fascinating, particularly Dolphus Raymond and B. B. Underwood. They both fit into the theme of appearances not fitting reality: Raymond is "fascinating" to Scout despite his "sinful" reputation, a wealthy man who fools the citizens of Maycomb into believing he is a mentally unstable drunkard. He is neither. Underwood, who "despises Negroes," nonetheless defends Tom (and Atticus) at the jail and later scolds the townspeople of Maycomb about the circumstances of Tom's death. Other interesting minor characters you may consider are Lula, the racist black woman who tries to prevent Jem and Scout from entering her "nigger church"; Little Chuck Little, Scout's brave classmate; Cousin Francis, Scout's obnoxious relative with gender issues; and the Misses Tutti and Frutti, another pair of author Harper Lee's eccentric women from TKAM.