In Harry Potter, there are two Orders of the Phoenix — the first one formed in the seventies, around the time Lord Voldemort and his minions began gaining power through force and violence. It was among the most influential combat entities in the First Wizarding War, a role reprised by its successor, which finally defeats the Dark Lord once and for all.
Numerous members from the original squad were unfortunately murdered in their heroic attempts to maintain peace in the Wizarding World, such as Fabian Prewett and James & Lily Potter, among others. The second did not fare better in terms of character death, but it is their strength that Harry relied on to complete his ultimate mission.
Harry may be the star of the show, but he is miles from being the most powerful magic-user in the world. In fact, it can be safely assumed that he never once achieves this title, even when he becomes a famous Auror.
As such, there are way too many talented witches and wizards in the Order for him to be ranked any higher, but it must be stated that it is the combination of Harry's various characteristics that make him a formidable opponent — the most critical of which is his willingness to sacrifice his life to protect his friends and loved ones. This generous nature of his is indirectly responsible for Voldemort's defeat at the Battle of Hogwarts.
There has never been any doubt about the inevitability of Hermione Granger's success, whether as the best student at school or the future Minister for Magic. She apparently could generate spells before the first year had begun, not to mention that she has often scored more than 100% in her exams.
She is a crucial part of the Order, perhaps not at the beginning, but certainly when she, Harry, and Ron have to go into hiding from Voldemort and the Death Eaters. Her role in preserving the Golden Trio cannot be understated.
From her capacity as mother to an unruly bunch of fiery-haired children, Molly's magical aptitude seemed limited to the Burrow and keeping the twins in line. However, The Deathly Hallows allowed Molly to demonstrate the true scope and intensity of her abilities.
Ginny's life being in Bellatrix's hands triggers something primal and overpowering in Molly Weasley, leading to the death of the most powerful Death Eater (who has, in the past, eluded the likes of Dumbledore).
Sirius Black's Animagus skill set notwithstanding, the man is as sharp and proficient as someone who never spent a dozen years locked away in Azkaban, with no access to a wand whatsoever.
He breaks in and out of Hogwarts under the noses of the professors, none of whom are easily fooled. In fact, it's possible to argue that Sirius may have not lost the battle with Bellatrix, because it's clear that he did not show restraint during his final moments. Excluding his infrequent bouts of carelessness, Sirius is a fine Order member.
Kingsley Shacklebolt ends up as the Minister for Magic immediately succeeding the defeat of the Dark Lord, so that's already saying something. He can, to an extent, handle the forces of darkness being thrown at him by Voldemort (however, he does require backup in this situation).
The Order has placed him in a number of significant positions, such as taking the place of the British Prime Minister's "bodyguard", or when he manages to escape five pursuing Death Eaters — bringing down two of them while flying.
Remus Lupin became an Order member considerably early, presumably a few years after he graduates Hogwarts. In the later timeline, he keeps Sirius and Snape from going at each other's throats, a hostile environment that most wizards would be terrified of entering.
He is, hands down, the most incredible Defence Against the Dark Arts teacher ever shown at Hogwarts, implying that his knowledge of the magical arts is considerably high. He may have died in the end, but his conviction and encouragement help Harry with his arduous journey.
Snape's level of magical aptitude surpassed his childhood bullies by a mile, but his dedication to the field of academia might have helped there. Also, he has picked up a horde of new skills from Lord Voldemort, such as flight. Speaking of the Dark Lord, one of the main reasons that he is unable to doubt Snape's intentions is because of the latter's incredible Occlumency abilities.
Besides, the Potion Master has saved the day on many an occasion, like assisting with Lupin's literal lunacy or medicating Dumbledore for a short duration after the Headmaster accidentally gets himself cursed by a Horcrux.
In his prime, it was believed that Moody was among the most remarkable Aurors of the era, an assumption that continues to hold true throughout the series as well.
His reputation precedes him, because it prevents Snape from mocking the DADA course in The Goblet of Fire, a reverence refused to the other one-year professors (Quirrell, Lockhart, Lupin, and Umbridge). Dueling aside, Moody's collection of enchanted objects, specifically those related to Dark Magic, suggests that the man is capable of far more than he lets on.
The most impressive thing about Professor McGonagall is not the exquisite control she wields over numerous forms of magic, nor is it her teaching skills or tactical knowledge. It is the fact that she, at her age, could take four simultaneous Stupefy jinxes in the chest and recover completely.
Further, Poppy Pomfrey furiously declares that she would have defeated all of them if she actually wanted to, a claim that can hardly be argued against. In any case, McGonagall's relevance is not downplayable, seeing as she's been part of the story since the very first chapter (appearing in her cat animagus form on the second page, to be specific).
Albus Dumbledore is by far the most powerful magic-user known in the Harry Potter universe, which automatically makes him the strongest among the Order as well. His spellwork is beyond description and has been since he was a student, given that even a Hogwarts examiner could not fathom the complexities of his magic.
Then there is the matter of the Elder Wand — it can be said that Dumbledore is certainly more effective while wielding it, but the fact remains that he defeated Grindelwald who had the benefit of the Elder Wand at the time. Most importantly, though, his presence has held the Wizarding World together for the longest time.
