My first reaction was, of course, that Atticus is the father dreams are made of. However, there is a debate to be made, and I shall help you make it.
The first question you should raise is "What does it mean to be an excellent father?" This varies from culture to culture, but many--including the one I was raised in--defines it as a father who looks out for his children's best interests and protects them, who takes a lively interest in their education and activities, who doesn't let them use filthy language, and who instills discipline and respect for elders in them.
Atticus fails on all these counts.
He doesn't seem focused on looking out for their best interests when it comes to protecting them from the social problems associated with the trial, and he fails to protect them on several occasions. They are daily subjected to being accused of being the children of a "nigger-lover" and other nasty remarks because he chooses to defend Tom Robinson, even when he knows what it will put his children through. (I just answered another question on eNotes this morning about the myriad points where the townsfolk accuse him of "lawin' for niggers" and such, so do a quick search for quotes and pages.)
He fails to protect them when he goes downtown to sit in front of the jail, awaiting the inevitable lynch mob. When they show up, he is unable to force them to leave. This situation could have easily swung in the other direction (as far as he knows), where the mob failed to "become human" (when Scout got Mr. Cunningham into conversation), and the children would have seen, up close and personal, the horrible things white people did to black people just for being black. They may well have been hurt in the process (Chapter 15). Again, he doesn't take Bob Ewell's threat that "he'd get him if it took the rest of his life" (247). Even when Ewell tries to break into the judge's house, Atticus doesn't take the threat seriously; it's as though he's living in a world of his own. Naturally, he isn't there when Ewell attacks Scout and Jem on the way from the Halloween pageant, and would have killed them had not Boo saved them.
He doesn't seem to take much of an interest in their education. Yes, Scout learned to read by sitting in her fathers lap and following his thumb, but he doesn't seem interested in their education in the book, other than to listen to their stories if they thought to tell him anything. He doesn't step in when he sees them playing "Boo Radley," even though he is well aware of it and should tell them to stop mocking the neighbors, as it's rude.
When Scout starts school, she begins a campaign to convince Atticus to let her stay at home because she hates it, and one of the things she does is use vulgarity. She says, "That's a damn story" to her Uncle Jack and later, "Pass the damn ham, please." Atticus doesn't nothing to Scout; he merely says, "Don't pay any attention to her Jack. Cal says she's been cussing fluently for a week now" (90). Uncle Jack later admonishes her in private for her use of such language--something we'd expect a good father to do, but Atticus doesn't.
Atticus doesn't instill any real discipline, as his children refuse to obey his orders when they feel like it. The most notable example of this is in the lynch mob scene, in which Jem will not go home when Atticus tells him to, and the men in the mob even laugh at him for being unable to control his own son.
Further, the children run wild in the neighborhood unsupervised, harassing the Radleys. They sneak into the trial, knowing their father wouldn't want them there, because--you can argue--they have no discipline and they lack adequate respect for him. Last but not least, he doesn't seem to put any effort into raising Scout to be a proper lady, as was expected at the time and actually crucial to her future social life, and doesn't actively support Aunt Alexandra's campaign to introduce her to the finer side of (feminine) life.
I hope that's enough to get your started!