MERLIN. Probably the most famous wizard in all of popular culture—yes, even eclipsing Harry Potter—Merlin's greatest feat was helping Arthur Pendragon find a very nice sword (Excalibur) and an even nicer cup (the Holy Grail).
According to records from the wizarding world, he is thought to have been among the most powerful wizards in human history. Merlin attended Hogwarts as a Slytherin and displayed a remarkable command of the magical arts. Some claim that he is the most potent wizard ever.
He may be the titular character, but Harry is far from the most powerful wizard in the wizarding world. Sure, he does eventually go on to take down the greatest dark wizard of all time, but his success comes more as a result of his mother's sacrifice and the support of his friends than it does his own brilliance.
Voldemort himself wasn't more powerful than Dumbledore, and his followers, including Bellatrix, definitely weren't. Don't get me wrong, she was dangerous, and committed horrible crimes, but she wasn't as strong as Albus. After all, she was ultimately defeated by Molly Weasley.
From using white fire to telekinesis, Gandalf's vast abilities make him by far the most powerful fictional wizard to date.
Lord Voldemort
Tom Marvolo Riddle started both Wizarding Wars, created the Death Eaters and killed Harry Potter's parents. We could go on – he was the most dangerous Dark wizard of all time – but we won't.
This is thanks to Flamel single-handedly created the eternal life-granting Philosopher's Stone. That's right, he made it all on his lonesome, without Dumbledore's help. If knowledge – even knowledge of an esoteric branch of magical study – is power, then Nicholas Flamel is easily more powerful than Dumbledore!
In his prime, Dumbledore truly had no equal who could best him in a one-on-one wizarding duel. Even though he might seem like an old man, his wizarding power is on a completely different level. Gandalf is probably the most powerful opponent he'd ever face, but Dumbledore wouldn't go down without a fight.
Severus Snape might have pretended to serve Voldemort (while in fact, as we all know, working as a double agent for Dumbledore), but there's almost no question he was more powerful than the Dark Lord.
The only one Grindelwald did seem to have genuine fear of appears to be Albus Dumbledore, as he quickly fled the scene after Ariana was killed trying to intervene in a duel he started, and later in life when he refrained from attacking England despite his possession of the Elder Wand out of fear of his former friend.
Avada Kedavra
Topping the list for the most powerful spells in the Harry Potter universe is no other than Avada Kedavra, Voldemort's favorite to instill his will and fear onto others. The spell itself is a play on Abra Kadabra and is easy to cast by just about anyone in the Wizarding World.
Barry Wee Willie Winkle (b. 14 August, 1236) was the oldest known wizard in the 1990s, when he reached the age of 755. To celebrate the occasion, he held a massive party, inviting every wizard and witch he had ever known, a total of thirty million people. It was one of the biggest parties in wizarding history.
Throughout the Harry Potter series, Albus Dumbledore, the beloved Headmaster of Hogwarts, proved time and time again that he was the smartest hero of the Wizarding World. His magic was so strong and his knowledge so extensive that he was the only wizard Voldemort was said to be afraid of.
Tom Riddle/Voldemort
Professor Dumbledore once said that Tom Riddle, a.k.a. Lord Voldemort, a.k.a. "He Who Must Not Be Named" (Ralph Fiennes/Hero Fiennes Tiffin/Christian Coulson), was the most brilliant student that Hogwarts has ever known.
He did not love anyone, and did not care to be loved. He felt nothing for his family, and only wanted his friends in school to rally around him because he wanted to hold power over them. Bellatrix Lestrange was obviously in love with him and worshiped him, but he only cared for her servitude, not her.
Grindelwald Is More Powerful and Cunning Than Voldemort
Grindelwald had more power in the Wizarding World up till the final moments of his life. He had more followers, and they were intensely loyal to him. He conquered more of the globe and could perform more advanced forms of magic.
When questioned by Bellatrix about his loyalties, Snape says he has been working for Voldemort ever since Voldemort's return, and explains his actions in the previous books in that light. He points out that gaining Dumbledore's trust and protection has kept him out of Azkaban and free to operate on Voldemort's behalf.
Regardless, Glorfindel remains one of the strongest characters in The Lord of the Rings, with powers similar to that of the Maiar after being, essentially, brought back from the dead to aid in the Third Age and the War of the Rings.
In terms of consistency, The Lord of the Rings had the definite advantage because the entire series was shot simultanously with a single director (Peter Jackson) in New Zealand. Over the course of the franchise, the changes in the characters' looks were done gradually and based on the progression of the story.
1. Michael Gambon. While he was obviously given the most time to develop the role, Michael Gambon truly embodied Dumbledore. Throughout his five appearances, Gambon hinted at the complexity within the character, showing how his relationship with Harry changes over time.
During book 7 is definitely Voldemort. By the time of the Cursed Child, it's probably Delphini. As for your other questions: Dumbledore was not powerful because of the Elder Wand. He took it from Grindelwald after defeating him in a duel (with a regular wand).
Voldemort intentionally made six Horcruxes, but when he used Avada Kedavra on Harry, he unintentionally created a seventh Horcrux. Instead of dying, Lily's love for Harry created a counter 'curse' known as Sacrificial Protection and saved Harry.