The Order of the Phoenix sees Hermione have some of her more brilliant (and impactful) ideas throughout the Harry Potter series - and her decisions here are, for the most part, incredible. Of course, Hermione is never perfect, and she does make some bad moves, but these are usually out of naivete more than anything - like her decision to hold the first DA meeting at the Hogs Head, rather than the Three Broomsticks. As Hagrid points out, the noisier, busier pub would make things harder to hear, and would be less suspicious!
Despite the occasional misstep, though, Hermione carries on being her usual brilliant self in this installment of the series, responsible for the creation of Dumbledore's Army, and for Umbridge finally being driven from Hogwarts.
Suggesting The Creation Of Dumbledore's Army...
By far, Hermione's best decision throughout The Order of the Phoenix is to create Dumbledore's Army. Harry may be the one teaching, but this is Hermione's brainchild, and it was a brilliant move, especially with the decision to keep meeting after Umbridge ensures that it would become against the rules.
Hermione may have claimed that the initial purpose was to help them with the exams (which they passed with flying colors), but this group became so much more than that. As Voldemort gained power again, the friendships formed and spells learned during the DA days kept together a core of resistance at Hogwarts, which no doubt saved real lives in the final years, and all because Hermione decided to form it this year.
... And Creating The DA Coins
Beyond simply creating the DA, Hermione managed to protect it with some impressive spellwork, and the most impressive was the DA coins. Inspired by the Death Eaters' Dark Marks, the coins were a great method of communicating, with changing numbers and the ability to heat up when something changed. The decision to use these to set times and dates was one that helped the DA exist during this movie, but more than that, it's revealed in the books that Neville uses them during the Battle of Hogwarts to call in reinforcements - something that may have made a huge difference in the end, if he hadn't been able to.
Getting Rita Skeeter To Interview Harry
Odious as Rita Skeeter is, it's Hermione who comes up with the idea to have her interview Harry, and it turns out to be a brilliant decision on her part. It enrages Umbridge, of course, but that is generally seen as a good thing! Beyond that, this is the interview that allows Harry to be heard, and to stand up against the lies and propaganda of the Ministry.
It wouldn't have been a minor d decision, either, given Hermione's history with Skeeter, but one that definitely paid off in the end - and which helped build the relationship with the Lovegoods, which would also become very useful later on, in learning about the Deathly Hallows.
Lying To Umbridge To Save Harry From The Cruciatus Curse
Hermione's lie to Umbridge was a great move on many levels - and impressive, because it was a series of split-second decisions, all made just to protect Harry. It was when Umbridge threatened to use the Cruciatus Curse on Harry that Hermione made her choice, leading Umbridge into the Forbidden Forest with a lie about a secret weapon, and in the end, it led to Umbridge being carried off by Centaurs, and Harry and Hermione free to go and rescue Sirius. It did nearly backfire, though, as the Centaurs realized that Hermione had hoped to use them for her own gain, and were definitely not happy about that situation! Thankfully, she made it out alive, but this decision could well have gone the other way, leading to her sharing Umbridge's fate.
Using Silencio At The Department Of Mysteries
All of the members of the DA who were at the Battle of the Department of Mysteries made some incredible decisions, especially around which spells to use, and how to protect themselves against the Death Eaters. Hermione's decision to use Silencio on Antonin Dolohov was a brilliant one - and one that may have saved the lives of her friends, as she was able to prevent him from calling out to the other Death Eaters and letting them know where Hermione was. In the confusion of a battle like this one, it's easy to forget how these small decisions matter, but they all do - and while large parts of the battle did not make it into the movies from the books, this was a great moment for Hermione, however short.