Draco may make some terrible decisions in The Half-Blood Prince, like letting the Death Eaters in, but he makes some good choices too.
The Half-Blood Prince marks what may be Draco's biggest struggle in the Harry Potter series - after years of being raised to feel superior and sure of himself, his family has fallen from favor and lives in terror of the Dark Lord (while still in his service). Draco himself has a choice to make - as a newly minted Death Eater, he has been charged with killing Dumbledore, and yet, he is still only a teenager. Watching him struggle to live up to his family legacy, please his father, and protect himself, despite clearly not wanting to become a killer, is heartbreaking to watch. Given how much of a pure bully and a villain he is in the earlier installments, it's an impressive shift for the character, and one that is centered around a key choice: is he a murderer and a Dark Wizard, or does he have good in him after all?
As well as this huge decision, Draco has many smaller choices to make throughout the Half-Blood Prince. And while some of these were clearly the wrong choice, or a misguided choice made by a lost teen, others were surprisingly solid. Whether they were made to protect his family and himself, or because he was truly a better wizard than fans have been led to think, these are some of his best moves in his sixth year.
This may not seem like a good decision, and compared to others, it is probably his least impressive choice that can still be called a 'good' one. However, there is merit to Draco's decision to get his way at Borgin & Burke's through threats and intimidation, as it is a key part of saving his life and that of his family.
Had Draco not threatened the store owners over the Vanishing Cabinet, they may have moved or sold it, and that would have done away with his only real plan to let the Death Eaters into the school. Had he not managed that, it's likely that Voldemort would have killed him - so in terms of self-preservation, this was a smart move! It's also in line with what a Malfoy and a Death Eater would do, which serves as another way to keep him safe, by blending in with the people that threaten him and his family.
In this year at Hogwarts, Harry becomes convinced that Draco is now a Death Eater (admittedly, he is correct on that point), and uses his Invisibility Cloak to hide in the Slytherin train compartment to spy on Draco and his friends. Draco, however, manages to figure out that Harry is there, and is able to stun him while he's still invisible and almost send him back to King's Cross station.
While his choice to also break Harry's nose was clearly an unacceptable violent outburst, the rest of this scene is actually fairly impressive. Of all the people that Harry has come across while invisible, Draco is one of the few to actually discover him - and not because of a slip of Harry's, but because he is observant enough to figure it out. Being able to Stun an invisible opponent is also fairly impressive, and something that not every wizard could manage. More than all that, this is an action that would be enough to put the majority of other students off the idea of spying on him, which would also help protect him and his plan. Of course, Harry isn't so easily deterred, but overall, it wasn't the worst move Draco has made.
Over the course of the series, it becomes clear that many witches and wizards at Hogwarts have been able to figure out the Room of Requirement, to some degree, but it's impressive that Draco manages to understand it as well as he does. Given that Hogwarts is repeatedly described as the safest place in the magical world, and as somewhere that Dumbledore is essentially untouchable, his choice to use the Room of Requirement as an entrance is actually incredibly smart.
Of course, the motivation behind this, like most of his actions, is obviously terrible - he is figuring out the Room of Requirement in order to follow through on a plan as a Death Eater to kill Dumbledore. And it's definitely arguable that the only way he was able to get this far was that Dumbledore was already dying, knew it, and had no intention of getting too far in Draco's way or of letting the boy actually kill him. However, there's still a lot here that suggests Draco is far smarter than the snobbish bully he may have seemed to be at first.
Once again, Draco shows surprising aptitude with this, but also impressive decision-making skills. His first attempts at killing Dumbledore have failed, and he makes the choice to try again, with something more devious and less likely to get him caught - something that does actually manage to succeed. His choices here also show his perseverance, and his ability to work through a problem logically. As Hermione points out back in the first book, when she comes up against Snape's logic puzzle, most wizards have absolutely no logical abilities, so Draco's choices here are all the more impressive.
Once again, it's hard to call this a purely 'good' decision, of course, as it leads to the death of Dumbledore (even if he was already dying), lets Death Eaters into the school, and generally wreaks havoc. However, as a decision for Draco, it manages to save his life, and keep his family safe, which is no small feat.
Finally, his most obvious best decision in this installment (and possibly in the entire series) is not to kill Dumbledore - even though this still ends in his death. He may have appeared to be responsible for his death, having let the Death Eaters into the school, but he wavered and held back when Dumbledore was disarmed, weak, and alone. In the end, it was Snape who killed him.
Draco's decision not to kill Dumbledore proves that he isn't the truly evil young man that he may have seemed to be at first, and that when it comes down to murder, even Draco will draw the line. This is an even more impressive decision given that he's been so clearly driven by fear for the safety of his family, and he knows that he may well be killed by Voldemort for this choice. In the end, Draco makes one decision that can only be considered the right one.