Some would say that Dolores Umbridge is the most evil character in Harry Potter, but really, it's between Gellert Grindelwald and Lord Voldemort. Generally, being worse at something means being poorer in performance, but that's not always the case.
Of all of the Death Eaters that appear in Harry Potter, there's no doubt that Bellatrix is the most evil. In addition to the fact that she was selflessly devoted to Voldemort and his mission (in fact, she was obsessed with him), she also showed that there was no depth of cruelty or malice to which she would not sink.
1. Albus Dumbledore. Image Via Warner Bros. Dumbledore (Richard Harris and Michael Gambon) may not have been the one to ultimately defeat Voldemort, but he was frequently cited as the only person Voldemort ever feared -- meaning he was the main rival of both of the most infamous Dark wizards in modern history.
In terms of body count, Voldemort is worse. He kills people personally, but also bears responsibility for the deaths that his followers, such as Bellatrix, carry out in his name. Umbridge never directly kills anyone, though she causes several deaths in an indirect way by sending innocent people to Azkaban.
Lord Voldemort is the scariest villain in Harry Potter, if only because he is the series' overreaching villain who successfully evades death more than once.
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. His horrific achievements are well known.
We would have to give the victory to Darth Vader. While the two dark lords seem perfectly matched on their villainy and power, Darth Vader's lightsaber and ability to immediately use the force to choke Voldemort, without chanting any spell would end this showdown in seconds.
Voldemort apparently believes nothing is worse than death; perhaps his greatest weakness is his inability to love. This is because his mother, Merope Gaunt, gave Tom Riddle Senior (Voldemort's father) a love potion. Because Voldemort was conceived under a love potion, he isn't able to feel or express love in his life.
Hermione Granger
Perhaps Hermione would have had some trouble against Voldemort in her early years, but as an adult witch, she could certainly give him a run for his money if not outright demolish the Dark Lord. Her power combined with her breadth of knowledge is tough to beat.
Bellatrix Lestrange Killed her own cousin, Sirius Black: She killed Sirius (who was her cousin) during the battle at the Department of Mysteries. She was remorseless and was cackling in glee after she had murdered Sirius.
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.
Voldemort's greatest fear is death and as a result a boggart takes the form of his own corpse. It is unsurprising that this is Voldemort's biggest fear. After all, he did split his soul up encasing it in various horcruxes just so to keep himself from dying.
The "most faithful servant" is Barty Crouch Jr., who has already been in place at Hogwarts working for Voldemort.
Throughout the Harry Potter series, we are led to believe that Dumbledore is one of the only people Voldemort actually fears; and for good reason. Albus Dumbledore is a well known and very powerful wizard in the Harry Potter series.
But Voldemort's killing curse rebounds, reflected by Harry's blood protection and the Elder Wand's allegiance to him, while Harry's Disarming charm wrenches the Elder Wand from Voldemort's hand to his. With all his Horcruxes destroyed, Voldemort dies instantly.
The Horcruxes Left Him Vulnerable & Weak
In creating these Horcruxes, he gave the ability for other witches and wizards to destroy him without ever laying a finger on him. Voldemort was a powerful wizard, comparable only to Albus Dumbledore or Grindelwald, rocking an awesome wand and a sense for world domination.
Thus, paradoxically, while Harry had to die before Voldemort could, Harry can't die while Voldemort lives. They have a double bond, with Voldemort's soul in Harry and Harry's blood in Voldemort. Dumbledore explains the mystery of why Harry's wand defeated Voldemort even when the latter had Lucius Malfoy's wand.
In Star Wars Legends and Canon, Galen Marek (Starkiller) is the only Jedi (other than Obi-Wan Kenobi) to put a beat down on Darth Vader/Anakin Skywalker.
Adam Warlock is a being of immense power. He has superhuman strength and agility, can manipulate energy beyond comprehension, and has a healing ability that would mean much of Vader's physical attacks would be unsuccessful. This fight would be over pretty quickly, and Warlock would be the victor, no question.
EU/Canon Palpatine wins easily. So much stronger/faster/versatile than Voldemort can ever hope to be.
History Of Avada Kedavra
The Killing Curse was created in the early Middle Ages by Dark witches and wizards. Primarily, the curse was used to quickly slay opponents during a duel.
Peter Pettigrew represents the worst Gryffindor in the entire Harry Potter series. Wormtail, as he was known among his friends, was thought to be a friend of the Marauders and a member of the Order of the Phoenix, but he was secretly working for Voldemort and committed the ultimate act of betrayal in the series.
Gellert Grindelwald was the world's most infamous dark wizard before Lord Voldemort. He immersed himself in the Dark Arts while he was studying at Durmstrang, one of the eight most prominent wizarding schools in the world.