The Dursleys are the worst sort of muggles imaginable, according to Professor McGonagall. They lied to Harry about his magical abilities, they lied to Harry about the death of his parents, and they forced Harry to spend a decent amount of his childhood in the cupboard under the stairs. Not only was this treatment bad enough, but their treatment was juxtaposed with their preferential treatment of Dudley, who had two bedrooms until Harry's second year at Hogwarts.
That being said, the Dursleys may have had some good moments to balance out the bad. After all, they weren't as evil as Voldemort or as malicious as Umbridge.
10 Jerk: Locking Hedwig In Her Cage
Hedwig was Harry's pet owl and companion. More than this, though, Hedwig was also his main connection to the wizarding world during his summerly exiles with the Dursleys. Owls such as Hedwig require space to fly and hunt. However, Vernon Dursley insisted that the snowy owl be locked in a cage during her time in Privet Drive.
Treating a snowy owl in this manner will only result in damage to the owl. This means that not only were the Dursleys practicing animal cruelty, but they were also doing so in order to make Harry miserable.
9 Nice: Dudley Tries to Reconcile With Harry
In 1997, Dudley started to try and reconcile with Harry. This was due to the dementor attack in the Order of the Phoenix. The attack from the dementor forced Dudley to see his own misdeeds, making the boy experience some self-reflection for the first time in his life.
As a consequence of this self-reflection, Dudley began to try and reconcile with Harry. The most notable example of this was when Dudley left a cup of tea outside Harry's door. Harry thought it was another prank from Dudley, but it was actually an attempt to reconcile.
8 Jerk: Mocking Cedric
At the start of the Order of the Phoenix, Dudley bumps into Harry at a playground while hanging out with his own friends. Rather than just saying hi or ignoring Harry, like a normal person, Dudley decides to mock Harry. Dudley brings up the fact that Harry has been talking in his sleep and shouting 'not Cedric'.
The fact that Harry is reliving the death of a friend is unknown to Dudley, who instead deploys the old homophobic 'is that your boyfriend' line.
7 Nice: Keeping In Touch With Harry
After the events of the Second Wizarding War, Dudley married a woman and had two children. In keeping with his redemption across the final two novels, Dudley and his family actually kept in touch with Harry. While it wasn't en incredibly close relationship, JK Rowling specified that they send each other Christmas cards and bring their children to play together if they were near one another, it was a huge development considering the way Dudley used to treat Harry.
JK Rowling had originally planned to have Dudley be at Platform nine and three quarters in the Deathly Hallows epilogue with a magical child but changed her mind due to the fact that Vernon's DNA would not allow for a magical child to be born.
6 Jerk: Locking Harry In The Cupboard Under The Stairs
The Dursleys lived in a four-bedroom house, yet still made Harry sleep in the cupboard under the stairs for over 10 years. This is insane when you take into account that they had two spare rooms that they could have easily given to Harry. Instead, the Dursleys decided to give Dudley a 'second bedroom.'
This forced Harry to sleep in a tiny cupboard with spiders and falling dust and dirt from the stairs above for over 10 years.
5 Nice: Agreed To Look After Harry Until He Was An Adult
If we look past the neglect and cruelty inflicted upon Harry during his time with the Dursleys, we can see that this family agreed to house a child that wasn't their own for 18 years. While their treatment of Harry was certainly abusive and disgusting, it was admirable of them to actually agree to keep this magical baby for 18 years and then keep their word all this time.
This, naturally, doesn't outweigh their treatment of Harry during his time with the Dursleys, but it is certainly important to remember.
4 Jerk: Putting Bars On His Window
Perhaps the Chamber of Secrets is the most overbearing the Dursleys had been to Harry. They insisted on locking Hedwig in a cage for the summer in order to prevent Harry from communicating with his friends from Hogwarts. If this was not enough, however, the Dursleys also placed iron bars over Harry's window in order to prevent him from escaping.
In putting the bars on his window, the Dursleys transformed their home from being a metaphorical prison for Harry to a literal prison. However, the bars on the window did lead to the epic Ford Anglia rescue...
3 Nice: Giving Harry Dudley's Second Bedroom
This is a fairly low bar, but considering their treatment of Harry up to this point, it was almost Christ-like from the Dursleys to eventually give Harry Dudley's second bedroom. Harry had lived in the cupboard under the stairs for over 10 years, with the spiders and the falling dust from the stairs above.
The fact that it took them so long, however, is unforgivable. As said earlier, it isn't exactly like they had no space available for Harry.
2 Jerk: Lying To Harry About His Parents' Death
In the first book and movie, the Sorcerer's Stone, it is revealed that Harry thought his parents died in a car crash. This is the reason that the Dursleys gave Harry for why he was living with them. This lie was told in order to hide the existence of the wizarding world, and magic in general, from Harry Potter. The Dursleys couldn't exactly conceal magic and say his parents were killed by an evil wizard.
This lie was heinous. Harry had the right to know how he died and that they died protecting him. If the Dursleys had successfully stopped Harry from learning he was a wizard, he may never have found out how his parents died.
1 Nice: Harry's Temperament
This may be a controversial addition to this list, but it is certainly arguable. Despite the fact that the Dursleys were undeniably horrible to Harry, they did raise Harry in such a way that he had good manners and a good mindset. It is because of this upbringing that Harry is such a likable, modest, and brave character.
Perhaps, if they had raised Harry like they had raised Dudley, Harry would become something closer to Dudley. This would almost certainly have placed Harry into Slytherin and would have certainly changed the whole result of the Harry Potter story.