Harry believes Dumbledore is "the best" (not necessarily restricted to teacher) because that is the image that Dumbledore creates for himself. Harry's first contact with the wizarding world is Hagrid, who tells him about his parents and offers him an escape from the Dursleys. Hagrid calls Dumbledore "The Greatest Wizard Who Ever Lived". Harry meats Ron on the train, his first friend of the same age. Ron and the Weasleys also think highly of Dumbledore. At school Dumbledore projects the benevolent all-knowing grandfatherly figure, and that is the way they remember him after school. He also defeated Grindelwald, and as a result got an Order of Merlin and influential positions at the Wizengamot and the ICW.
So in short, many people think highly of Dumbledore and tell Harry. He seems to know the answers to everything, even if he doesn't share them.
Unfortunately, looking at the facts shows that this respect is not justified, in effect Harry (like many others) is brainwashed by Dumbledore.
To those who downvoted this answers, doesn't it contain enough to support it, or do you just not like the conclusion? If you don't like the conclusion, can you point out Dumbledore's actions that contradict this conclusion?
Dumbledore is never Harry's teacher (the "lessons" about Voldemort are not lessons, they can be called background information for Harry's mission, although stretching them over the year was a waste of time). So the statement is more likely a reflection of Harry's respect and admiration for Dumbledore, but that is the result of Dumbledore's manipulations. Unfortunately Harry never learned to see behind Dumbledore's mask and recognized that he has been Dumbledore's pawn his whole life, so he names an innocent child Albus Severus.
At the end of the first book, Harry suspects that Dumbledore knows most of what happens at Hogwarts and wanted them to go after the stone. This might well be true, but it doesn't make Dumbledore a good teacher. On the contrary, it means that he willfully endangered the students.
It's one thing to let children explore and learn from their experience in a safe setting and something completely different to throw them into mortal danger and see what happens. Voldemort would have escaped with the stone if he had stunned Harry before trying to get the stone. Harry could have died on many occasions. And even the prophecy wouldn't protect him from being killed by Voldemort.
Dumbledore is a failure as a headmaster. Half the staff don't teach and/or abuses the students.
- Filch openly speaks about torturing students.
- History is tought by a ghost who puts everybody to sleep.
- Muggle studies is more than a century outdated.
- Defense has mostly poor teachers.
- Trelawni is a fraud, or maybe Divination just isn't teachable. Every year she predicts the death of a student, not just in Harry's year. Dumbledore himself wanted to get rid of the subject, it seems he just hired her to keep her out of Voldemort's reach.
- Hagrid may be a nice guy, but that doesn't make him a teacher.
- And of course Snape, who both doesn't teach and abuses students. Not just Harry, but also Neville, Hermione and basically everybody not from Slytherin. This one of the greater failures against the "greater good". Potions is needed at least for Aurors, Healers, and of course future Potions Masters. Snape's unwillingness to teach reduces the number of Aurors and Healers, at least those outside Slytherin, when Dumbledore strongly suspects that Voldemort will return and a new war may start.
- Dumbledore allows the Slytherins to abuse other students. This shows them that it is perfectly acceptable to bully other people, and later torture and kill them.
- Dumbledore ignores the safety of his students.
- The stone is kept in the school as bait for Voldemort.
- A Cerberus is behind a simple locked door that any first year can open.
- A troll is reported in the dungeons, and Dumbledore sends half the school into the dungeons.
- As punishment for being awake at night and missing their sleep, four students are sent out, missing another night of sleep. And they have to go to the forbidden forest, which is always dangerous, but now they shall search alone for something strong enough and willing to kill unicorns.
- Something petrifies students. What does Dumbledore? He ignores the threat and hopes for the best.
- While Draco tries to kill Dumbledore, two students almost die (Katie Bell and Ron). Yet for Dumbledore it is more important whether Draco will try to kill him in the and than whether other students are killed in Draco's attempts.
- In general, Dumbledore has other priorities for the school, and the safety and education of the students are second to that.
Dumbledore is a failure as part of the government. He is the Chief Warlock of the Wizengamot, the legislative and judicial body of the government. In addition, many people listen to his suggestions, which gives him additional influence. Yet most Death Eaters remained free and in power, ready to support Voldemort again. On the other hand, he fails to make sure Sirius gets a trial. Even if everybody believed he was guilty, and even if everybody was drunk for a month to celebrate the defeat of Voldemort, Sirius should have been questioned with Veritaserum, just to know what else he did and what other Death Eaters he might know. As this was done with Karkaroff, it's not like nobody thought about that.
Dumbledore is a failure regarding Harry. Most of Harry's suffering can be traced back to Dumbledore. Just from the first chapter of the first book:
- Dumbledore doesn't even have the decency to ring the bell end tell Petunia in person that her sister is dead and she is expected to raise Harry. Instead he placed a child on the doorstep like a bottle of milk on a cold November night. One might argue that he knew that Petunia wouldn't want Harry, which brings us to the next point.
In OOTP Dumbledore admitted that he knew that he was "condemning Harry to ten dark and difficult years" (OOTP chapter 37) even before he placed him there.
Well – not quite whole. You had suffered. I knew you would when I left you on your aunt and uncle’s doorstep. I knew I was condemning you to ten dark and difficult years.
Mrs. Figg was watching Harry, so she confirmed to Dumbledore that Harry was abused, as Dumbledore knew he would. As we can see from the Dursleys' reaction to the letter addressed to the cupboard under the stairs, they immediately moved Harry to the second bedroom. So a few letters to the Dursleys (We are watching you, Harry doesn't get enough to eat, he is mistreated) would have improved Harry's living conditions.
- Dumbledore says that he would not remove Harry's scar (Voldemort's horcrux) even if he could. So Dumbledore wants Harry to die even if there was a way to remove the horcrux.
So as Dumbledore knew about Harry's living conditions and could easily have improved them, but didn't, we have to assume that it was his intention that the Dursleys treated Harry the way they did. He forces Harry to go back every summer, then generously allows Harry to leave early, and Harry is grateful for the escape. On several occasions Dumbledore claims that he wanted Harry to have a childhood (end of book 1 and 5), but he knew that Harry would be treated worse than Tom Riddle at the orphanage.
Harry is raised to unconsciously accept the blame for everything that goes wrong, to consider himself and his life worthless, so that he would be willing to die when Dumbledore says so. Harry is raised "as a pig for slaughter" (DH) as Snape notes, and Dumbledore doesn't deny but tries to distract.
There are hints that Dumbledore might have suspected that Harry might survive being killed by Voldemort after Voldemort used Harry's blood. But that was obviously not something he knew when he placed Harry with the Durlsleys, so his plan that Harry should be abused and willing to die predates that.
For a fleeting instant, Harry thought he saw a gleam of something like triumph in Dumbledore's eyes. GOF
Dumbledore is a failure regarding the war against Voldemort. He has a prophecy that "The one with the power to vanquish approaches ... and either must die at the hand of the other".
This can mean that Harry is one of many who can defeat Voldemort, or more likely that Harry is the only one, because the prophecy doesn't say that Voldemort won't die before he kills Harry and then someone else will be successful, it talks about the power he knows not, so while the outcome isn't sure, Harry should at least have some advantage. And yet, Dumbledore's plan is that Harry should be the one who dies and not Voldemort. And that was his plan from the moment he decided to place Harry with the Dursleys, even before he saw Harry or the scar.
In the end, Harry doesn't win because of Dumbledore's plans, he wins despite them. According to Dumbledore's plan, the Elder Wand would either have no master, or Snape. As Snape was killed by Voldemort, in their final confrontation the master of the wand would be either nobody or Voldemort, in both cases Voldemort would win.
Harry doesn't die by Voldemort's hand, Voldemort doesn't die by Harry's hand, he dies from his own curse. So the prophecy is either wrong, or it was fulfilled when Voldemort met baby Harry. After that, only one of them could live, and Voldemort was more or less dead for a decade after that encounter.
Of course Dumbledore is not stupid, so he behaved in accordance with his goals. It's just that his goals were not in the interest of Harry, the school or the wizarding world.
But Dumbledore is very good at manipulating others and create the wise and caring image of himself. He is the only worthy Slytherin in the whole series. Harry was supposed to be cunning enough for Slytherin, but it seems being sorted into Gryffindor surgically removed all his cunning. Draco is just plain stupid. Even Voldemort, the supposed heir of Slytherin, is stupid. He hit Harry two times with the killing course and Harry is still not dead. After he used the killing curse two time on Harry and Harry still lives, Voldemort tries it again even after Harry explains to him why it would not work (the classical villain mistake that gives the good guy the time to escape).
So to conclude, Harry's admiration for Dumbledore is not justified.