The members of Dumbledore's Army are, almost by definition, some of the bravest students at Hogwarts. These are the students willing to risk breaking the rules to stand up to Umbridge - and given her predilection for literally torturing students as punishment, that's no small matter!
These are the students willing to fight, and willing to defy the rules to prepare for war against Voldemort. While they are all brave, though, some are certainly a little more courageous than others. It's clear from the start that while many DA members are there because they expect to do battle with the Death Eaters, others are just a bit curious, or even more concerned about a good grade than anything!
Marietta Edgecombe
Marietta Edgecombe is one of the many characters that didn't make it from the Harry Potter books to the films - but in the books, she is easily the least brave member of the DA. In fact, she didn't really want to be a part of it at all, but was pressured into joining by Cho.
She is also the one who then betrays the DA, turning them in to Umbridge in order to try and avoid trouble (and out of fear that her mother's job may be in danger if she doesn't). Her willingness to turn in her friends because of a vague possibility that something bad might happen is an act of self-preservation, not bravery.
Cho Chang
Cho Chang has excellent intentions in joining Dumbledore's Army, her resolution hardened by the death of Cedric. However, her good intentions fall apart over time. While she works hard in the DA, she doesn't keep her coin in her pocket, and so isn't involved in the fighting the night Dumbledore died, and after Marietta betrays them, Cho is not brave enough to end the friendship. Instead, she steps back from the DA.
Cho does fight in the final battle of Hogwarts, but beyond this, it seems that her involvement is motivated by brave ideas, but not backed up by brave action. It's also worth noting that in the movies, Cho is the one to betray the group to Umbridge - but under the influence of Veritaserum.
Katie Bell
Katie Bell is an excellent Quidditch player, and a staunch ally of Harry's - she joins the DA happily, and works hard to become a better fighter in it. However, she is better known as the student who was accidentally cursed by a necklace while in Hogsmeade, and who spent most of the next several months in hospital recovering.
She does fight in the Battle of Hogwarts, and proves herself brave in how she deals with injuries and setbacks - whether minor (like Quidditch injuries) or major (the cursed necklace). However, as Katie is still a very minor character in the series, it's impossible to tell how her bravery stacks up against some of the more developed members of the DA.
Colin Creevey
Colin Creevey displays an impressive amount of bravery - even if he is also slightly irritating, especially to Harry. Colin is best known for being the annoying young student who loves to follow Harry around and takes photos of him, but in later years, he is also a member of the DA, and in the Battle of Hogwarts, he sneaks back into the castle in order to fight.
In a small way, it is brave of Colin to be as open about his excitement and photography as he is, in a school where he is likely made fun of for it. But it's his time in the DA, and his decision to disobey orders to actively put himself in danger and battle Voldemort that make him even more courageous. Tragically, Colin dies in the battle, but if he had lived, he may have proven his bravery even more.
Luna Lovegood
Luna is definitely one of the braver characters in the series, and is always willing to stand up and fight against the Death Eaters, but her bravery is quieter than that of most of the DA. Unlike Harry and his friends, she doesn't rush off to fight at the first opportunity, and tends to take most things with an optimistic passivity. Even when she is kidnapped by Death Eaters, she bears it with a quiet calm.
Of course, this is her specific kind of bravery - and it takes courage to stay so calm when in danger. She is also front and center in battles, and clearly doesn't lack that kind of bravery in action, either.
Ron Weasley
While Ron is certainly one of the braver characters in the franchise, as one of the Golden Trio, he's not as brave as he might seem at first. His willingness to go with his friends into the face of danger (every school year, too) is definitely the courage that makes him a Gryffindor.
However, he has his moments of cowardice, too - moreso than Harry and Hermione. His struggles when hunting for Horcruxes, and his eventual running away from that quest do not paint him in the most courageous light (even though he comes back, in the end). And often, when the Golden Trio are in danger, Ron is the most afraid. Of course, a willingness to continue forward even in the face of great fear is its own type of bravery, too.
Ginny Weasley
Almost all of the Weasleys are known for their bravery, and all are in either the Order of the Phoenix or Dumbledore's Army (or both) during the War. Ginny, however, deserves special mention for her own bravery throughout the series. After being possessed by Voldemort (or by part of his soul), she becomes a devoted fighter against him and the Death Eaters - joining the DA, and then staying at Hogwarts to protect other students, and finally fighting in the Battle of Hogwarts as well.
While she doesn't get the same chances to show her bravery as Ron does, being a comparatively smaller character in the franchise, Ginny may well be braver than her brother. Unlike Ron, she is never seen backing down or showing a more cowardly side, and is always front and center in the fray.
Hermione Granger
Hermione is, without a doubt, one of the bravest characters in the franchise - and while she is often frightened, she never backs down from doing what is right. She is the one to suggest starting the DA, even though it puts them in Umbridge's crosshairs, and of course, she is willing to remove herself from her parents' memories and go hunting Horcruxes with Harry and Ron. She even heads into the Ministry and Gringotts on separate occasions, putting herself in the most dangerous position of the Golden Trio.
While Hermione is incredibly brave, however, she doesn't quite face as much as Harry does. That's not to say that if Hermione was the Chosen One, she wouldn't have been just as courageous, but that in the series, she doesn't show that courage quite as much.
Harry Potter
Harry is the leader of Dumbledore's Army, and the reason for this is that by the time it is formed, he has already faced down more danger than all the other members. Harry faces off against all the Dementors of Azkaban that were stationed at Hogwarts, against the Basilisk, Voldemort, giant spiders, and of course, the awful treatment he endures with his family. He never fails to do what's right, even when it's difficult.
Harry was, of course, even brave enough to face death, thanks to the Resurrection Stone. There is absolutely no doubt of his bravery, although there is one member of the DA who may be braver still... Neville Longbottom. Becuase unlike Neville, Harry had Ron and Hermione with him at all times, special attention from Dumbledore and the other professors, magic that he knew would protect him against Voldemort, the Deathly Hallows to help him, and of course, the knowledge that he was the Chosen One.
Neville Longbottom
Neville Longbottom is one of the bravest, and often most underrated, characters in the Harry Potter franchise. He often proves himself as willing to face danger as Harry does, but without the kind of support system (emotional or magical) that the Boy Who Lived enjoys. Instead, Neville has only a second-hand wand and a disapproving grandmother, and yet he is willing to stand up for what he believes is right, right from the first installment.
In the final years of the franchise, he stays at Hogwarts to actively protect younger students, and in the Battle of Hogwarts when everyone else assumed hope was lost, he still stepped up, willing to face Voldemort alone.