Sirius Black was the godfather of the main character in Harry Potter and although he was a good man, he was very reckless and made some horrible decisions along with the great ones. After a presumably very abusive upbringing in his Pure-blood family as well as a twelve-year stint in prison that he did not deserve, his judgment was often questionable — so he can't really be blamed.
And after all, he did his best, and seemed to truly care about his godson and act only with him in mind.
Worst: Making Peter Pettigrew The Potters' Secret Keeper
Sirius thought he was doing the right thing. Voldemort was going to come after him and torture him for the Potters' location, and Sirius wanted to avoid any possible chance of him breaking. He passed the role of Secret Keeper to Peter and fled — and immediately, Peter sold them out.
He might have had good intentions, but it was what led to the Potters dying and Harry growing up an orphan. He did beat himself up about this a lot though.
Best: Breaking Out Of Azkaban
Breaking out of prison isn't often a good decision, but Sirius was doing it to save Harry's life. He slipped past the dementors as a dog, swum to shore, traveled all the way to Hogwarts, and revealed Peter Pettigrew to be hiding out in Harry's dormitory as a rat. It took some serious guts and determination to make it that far on weak health, and it was the beginning of rebuilding his relationship with Harry. Harry really needed his godfather by that point.
Worst: Going After Pettigrew Himself
When Pettigrew was first revealed to be a traitor, Sirius hunted him down and tried to kill him. It led to Peter escaping and Sirius being imprisoned, so not a great result.
Although it's hard to blame him for his gut reaction and many people would have done the same, it was certainly a bad decision because it got him twelve years in prison and Peter was able to scurry away and eventually bring the Dark Lord back at the end of Goblet of Fire.
Best: Joining The Order Of The Phoenix
Sirius grew up in a family that loved Pure-blood culture, were very prejudiced against Muggles, and brought their sons up to be the same way. Amazingly, Sirius managed to break away from this ideology and became a voice against inequality. He joined Dumbledore's secret society against dark magic, the Order of the Phoenix and considering he was turning against his family, this is seriously impressive.
Worst: Abusing Kreacher
One habit Sirius did take from his parents — or at least his mother — was how he treated house-elves.
Despite telling Ron he should "look at how a man treats his inferiors" to get the measure of him, he treated house-elves very poorly and was even physically abusive to Kreacher at points. He seemed to consider them sub-human and refused to listen to Hermione's reasoning. It was a bad decision because Kreacher found ways to leave the house, taking Sirius' snaps to, "get out!" very literally. Kreacher ended up being the reason for Sirius' death.
Best: Gifting Harry A Firebolt
Gifting Harry a Firebolt put him on the Slytherin's level in Quidditch and probably meant a huge deal to his godson. Sure, Harry was never a materialistic person, but he never got nice gifts until he was adopted by the Weasleys and Molly made sure he had a nice sweater every Christmas. Sirius spending that much money on him probably made Harry feel great.
Worst: Rushing To The Ministry To Save Harry
Although Sirius, once again, had good intentions when he rushed to save Harry at the Ministry, it led to his death. He was killed by his own cousin, Bellatrix Lestrange, and if he had stayed at home like he was told, he'd have stayed alive.
But Sirius was never a man who could sit back and let someone he loved be in danger. He also simply wasn't good at stepping back.
Best: Offering Up Grimmauld Place As Headquarters
Not only did Grimmauld Place provide the Order of the Phoenix with some suitable headquarters, it gave the trio a place to live for a while in Deathly Hallows (until Hermione accidentally led Yaxley to it). It was very selfless of Sirius to give it up — not because he cared about the house, but because he was trapping himself there by letting the Order use it. But it did come in very handy.
Worst: Sparing Pettigrew's Life
Sirius sparing Peter's life in Prisoner of Azkaban was nice and probably good for his mental wellbeing, but Pettigrew immediately escaped and brought Voldemort back. That would all have been prevented (or at least very delayed) if Sirius had gone ahead with his murder.
It was a noble decision, but it had terrible consequences.
Best: Asking Harry To Move In With Him
Asking Harry to move in with him seemed pointless in the end, since Sirius died before it could happen. But it let a lonely thirteen-year-old know that although he was neglected by his blood family, he had other family who actually wanted him around. Harry deeply and understandably hated the Dursleys, and it was a great move to assure him that his godfather wanted him around and loved him. It gave Harry a boost he so desperately needed.