In Harry Potter, the main character isn't exactly raised by a wholesome family. Left on the doorstep of his aunt, uncle, and cousin as a baby, he's neglected and downright abused by the people who were supposed to protect him. Miraculously, he still turned out to be a good person — the power of James and Lily's genes! — but the people around him were pretty awful.
They made some truly awful decisions throughout the course of the series, and even their good ones were bare minimum.
Worst: Abusing Harry
Easily the worst decision made by the Dursleys was the one to treat Harry terribly. He lived in a cupboard under the stairs, was starved, given secondhand clothes (when they could afford new ones just fine) in comparison to his cousin's good quality possessions, and was overall utterly bullied and put down at every opportunity. It was a conscious choice for them to do this to their nephew, and there's no excusing it.
Best: Taking Him In
They did save his life by taking him in. Due to the magical protection of being family — in the barest sense of the word — Harry was untouchable by Voldemort in Privet Drive. If they hadn't taken him in, he might well have been hunted down and kidnapped by Death Eaters for Voldemort to kill, but his life was never in danger there.
Even if his general wellbeing and mental health was.
Worst: Avoiding The Hogwarts Letters
When Harry began to get Hogwarts letters inviting him to the school, Vernon Dursley in particular was extremely against letting him go. He hid the letters from Harry and, when they became unavoidable, drove them to a remote location and deliberately taunted Harry.
Not only was this cruel, it didn't work. Hagrid hunted them down anyway and Dudley ended up with a pig's tail for his trouble.
Best: Letting Harry Go... Eventually
After all that, they did let Harry go to Hogwarts — and even dropped him off at King's Cross station. Of course, any good family would gladly do this, but in comparison to their other terrible decisions... this was one thing they actually did that wasn't awful. Even if they did strand him at the train station when he couldn't find the platform.
Worst: Locking Harry In His Room
Harry ended up with Dudley's second bedroom in Chamber of Secrets, though the joy of it wasn't to last long. Uncle Vernon put bars on his window and they slid food through a catflap, so little of it that Harry was clearly starving. It resulted in quite a scene when Ron, Fred, and George Weasley showed up to rip the bars off the window and drag him to freedom.
Did none of the neighbors think of questioning the prison bars on a preteen's window?
Best: Taking His Warning About Sirius To Heart
In Prisoner of Azkaban, Harry leaves the school year with a jaunty warning about his serial killer godfather to the Dursleys. This works in his favor. They're so terrified that they leave him alone, and although he's largely ignored and isolated for the summer, this suits him just fine until he can get back to the Weasleys.
Worst: Inviting Aunt Marge To The House
In the same book though, Vernon invites his sister over. Aunt Marge is a horrible woman who makes derogatory comments about Harry and his parents and seems, if possible, to be an even worse person than her brother.
It was a poor decision all round. Eventually Harry got so agitated by her remarks that he performed accidental magic and blew her up like a balloon.
Not recommended in the Wizarding World, but deserved.
Best: Letting Harry Go To The Quidditch World Cup
In Goblet of Fire, Harry asked to go the Quidditch World Cup with the Weasleys. Vernon almost seemed to be initially against it — though why, when he'd get rid of Harry for most of the summer — but let him go. It was a good decision, since the Weasleys would have turned up anyway and would probably have been followed by a vengeful Sirius Black.
Worst: Never Giving Harry A Chance
Overall though, the worst thing they did was never give Harry a chance.
Harry was always a good person — hot-headed, certainly, but brave and loyal, willing to die for his friends. They could have had a real bond with the savior of the Wizarding World but were too deep into their own prejudices to give him a chance.
Best: Leaving While They Still Could
In Deathly Hallows, they left while they still could, although Vernon wasn't keen on the idea. It was a solid choice. Voldemort would surely have hunted them down and tortured them for information about Harry and since Vernon could barely look Hagrid in the eye without trembling, he would have been no match in of magical power or courage for Lord Voldemort. Vernon might have grumbled about leaving his house behind, but he saved his own life and his family's by agreeing to it, however reluctantly.
