The Vampire Diaries began a mythology of vampires, werewolves, and witches. That mythology was continued in its two spin-offs: The Originals and Legacies. While the central supernatural character was the vampire, the vampires couldn't have existed without the help of the witches. The witches created daylight rings to allow vampires to exist in the day like ordinary people. Further back, we find that the original vampires were created by a witch, their mother.
Throughout the series, there have been numerous witches, especially in The Originals. Sometimes serving as a foe, sometimes as a friend, and sometimes as neutral, their presence has always been important in this universe. Regardless of the many witches introduced, certain ones stand out more in terms of their power and their importance.
Kai
Kai is introduced in Season 6 of The Vampire Diaries when Bonnie and Damon go to a prison world. Although they don't trust him, they don't realize he is a Gemini witch to be feared. Kai is a siphoner, meaning that he can absorb other magical energies and use them as his own. He also is a psychopath, and tried to kill all of his siblings in 1994. This act led to his coven creating the prison world so that the Gemini coven would be safe.
Though a villain, he is a powerful witch. He links Bonnie and Elena's life in a sleeping beauty spell. While Bonnie is alive, Elena "sleeps." Only Bonnie's death will awaken Elena. Thankfully, Bonnie and friends eventually find a solution, but for several episodes his spell is rock solid. Kai is also difficult to kill, coming back from death. Now, he is in another prison world, still a threat to his nieces, Josie and Lizzie.
Vincent
The Originals has many witches that could make this list (and quite a few that will), but Vincent isn't one to take lightly. The ancestors chose him to continue as the leader, and initially, he didn't want that responsibility. However, he grows to accept it, builds and protects the witch coven, and is respected by even Klaus, an original vampire. Freya (an even more powerful witch) often seeks out Vincent for help. He is definitely someone you want to be on your side.
Hope
Hope, Klaus and Hayley's daughter, is first introduced to us in The Originals. She is now the main heroine in Legacies. A triple threat, Hope is made of witch, werewolf, and vampire. The granddaughter of Esther and a first born daughter, she is very powerful. In The Originals, she is so powerful that takes the evil Hollow force from her father and his siblings into herself. Although her thought is to unite her family, the Hollow proves to be even too strong even for Hope. Clearly, Hope has much to learn, but her ability at magic make her seem a natural.
Valerie
Valerie, Stefan's former girlfriend and current heretic, seems the nicer heretic when she spells Caroline's skin to feel like vervain (poison to vampires) so that the other heretics stop torturing Caroline. This is a pretty powerful spell, and also serves to keep Caroline and Stefan apart (currently dating). Valerie still holds a torch for Stefan.
When the twins, Josie and Lizzie, prove themselves to be siphoners while in Caroline's womb, Valerie helps out. The babies, ready to be born, pull magic from Caroline, starting to kill her. Valerie holds their power back, allowing them to be born and Caroline to live.
Josie and Lizzie
At this point, it is hard to separate the current Gemini witches. Since they are twins, their power works the best and strongest when they are together. The twins have been on all three shows.
As children, they help Bonnie and themselves survive a fire in The Vampire Diaries. They also break a powerful spell in order to get Damon and Enzo out of a dangerous situation. Also, in The Vampire Diaries, they create Uncle Kai's current prison world. All this they did as children. Who knows what more we will see them do in Legacies Season 2.
Davina
Out of the four Harvest Girls in The Originals, Davina proves to be the most powerful, despite being neglected by the ancestors. Davina is the one to break Klaus's sire bond. Now, the vampires he sired are now free to kill him since killing him won't mean killing themselves. Davina also brings Kol back from the dead. Though a young witch, she isn't one to give up, especially when it comes to those who have wronged her and those she loves. You wouldn't want to get on her bad side.
Bonnie
From the powerful Bennett family, Bonnie learns of her magic through Gram Bennett, another powerful witch, in The Vampire Diaries. Bonnie becomes a strong witch, sacrifices herself for others many times, and learns to adapt to situations.
At one point, she even becomes the gate that supernatural beings have to pass through in order to get to the afterlife. At another point, when she took the hunter power from Rayna, she makes herself sleep so that she can't hurt those she loves. Bonnie's magical power changes throughout the series, at times it is strong and at times faint. However, she always figures out a way to teach herself the magic again or to adapt to any situation.
Freya
As Esther's eldest born daughter, Freya is born powerful and full of potential. We find out that Esther gave Freya to her sister, Dahlia, in The Originals. Though unhappy, Freya becomes even more powerful through Dahlia's tutoring. They are linked for most of Freya's life, their magic connected.
Freya is able to save her siblings, even halting the poison that is killing Elijah. She also stays immortal, mostly. Freya has lots of knowledge about magic, and she is now serving as a teacher to Hope.
Esther
Esther is considered the original witch; she is the witch that made the first vampires. We learn about this in The Vampire Diaries and The Originals. Although dead, Esther is able to bring herself back to life by becoming a New Orleans witch in The Originals. She is able to inhabit other witches' bodies and to bring her sons' souls back to life (Fin and Kol). She seems the most powerful witch of this universe. That is, until we met her older sister, Dahlia.
Dahlia
Dahlia appears to be the unbeatable foe in The Originals. Even Freya (a powerful witch) and Esther (a more powerful witch) fear Dahlia. When Dahlia comes to New Orleans, she makes every person on the street stare at Freya, as if she is looking at Freya through their eyes. Dahlia also makes the white oak (that which can kill or harm the Mickaelsons) turn into dust, forcing the debris down the original vampires' throats to kill them. If it weren't for Esther's sacrifice, Dahlia would have been unstoppable and ruled The Vampire Diaries' universe.