One of the best things about the Star Wars Expanded Universe becoming Legends and being disregarded as far as canon goes is that it wiped the slate clean from the insane amount of characters who were far too powerful. There were loads of characters and beings whose Force abilities were almost beyond imagination; while this resulted in a decrease in meaningfulness, it led to awesomeness too.
Some of the best and most important characters in Legends were overpowered Force users, whether they be Jedi, Sith, a flip-flop between the two, or Force users who transcend that petty rivalry and are of higher standing in the Living Force.
Anakin Skywalker
Many Legends characters are more powerful than Anakin/Darth Vader, but clearly he is still one of the most powerful Force-wielders in Star Wars history.
Anakin never fulfilled his potential, but even in Legends, he was born the most naturally powerful mortal, with an unimaginably high midi-chlorian count. It may be unfair to place Anakin here based on his natural ability and potential. However, even when he lost a lot of his abilities with his limbs, as Vader, he was still mighty, a genius with a lightsaber, and with the Force. With how powerful he was at that point, despite having lost so much of his Force potential, he has to be classed as overpowered.
Galen Marek/Starkiller
The protagonist of the beloved Force Unleashed game is another character who is not as powerful as the likes of Darth Bane, for example, but who is so overpowered.
At such a young age, Marek (and his clone Starkiller) was capable of Force abilities most his age could only dream of, with a natural presence in the Force that made Vader believe he was a Jedi Master when he was only a child, and on top of that, he even bested Vader in battle, an accomplishment very, very few can truthfully claim.
Revan
Darth Revan is a phenomenon like few others in Legends, beloved by an incredible amount of fans who want to see his story retold in canon.
Darth Bane himself considered Revan the most powerful Sith to ever live; that is as good a testament as any to the power of Revan, who was a master of telekinesis, as well as a master of both the ways of the light side and the dark side. His legacy far transcended his life, appearing as a Force ghost, splintered in the Force into two separate beings; the light and dark version. Widely considered the most powerful mortal of his time, there is no doubt Revan is overpowered as well as being a fascinating, excellent character.
Jacen Solo
Many hardcore Legends fans take issue with Kylo Ren, despite him clearly being one of the best characters of Disney's new era of Star Wars, due to him simply not being Jacen Solo, another incredibly popular Legends character.
Son of Han Solo and Leia Organa, Jacen Solo grew to be both a powerful Jedi and a powerful Sith as Darth Caedus. Caedus had a slew of abilities, and many moments highlight his power. However, if there is one thing that shows off how overpowered the character is, it is his ability to flow-walk, an ability that allowed Solo to experience both the future and the past in time, having a special incline for what is to come. He could also Force project brilliantly and teleport objects through Aing-Tii.
Darth Nihlus
It is arguably unfair to classify Darth Nihilus as a Force user on this list, as he was a wound in the Force, becoming so after surviving the unleashing of the Mass Shadow Generator on Malachor V.
What makes Nihilus so overpowered are his goals; he lives after that event solely to satiate his hunger, his need for Force energy. Nihilus literally consumed planets, devouring the Force power and energy from it and its inhabitants, one of the scariest shows of power from a Sith Lord ever.
Darth Sidious
Probably the most powerful Sith in all of Legends, that does not make Darth Sidious the most overpowered, but certainly puts him up there, with his power in Legends being far wilder than that in canon.
In Legends, Sidious is an embodiment of the Dark Side, unmatched in so many powers, able to conjure up Force storms to wipe out entire fleets, able to survive long after death, jumping from body to body, retaining all of his ability in the Force, and being more of a God than a mortal at some point.
The Sith Emperor (Vitiate)
Known as Darth Vitiate, Darth Tenebrae, the Sith Emperor, and as Valkorian, this powerful being was one of the strongest ever to grace the Legends timeline.
Deemed Lord Vitiate at only thirteen by Marka Ragnos, through experimentation in the Dark Side and the ability to possess and take over other's bodies, Vitiate manipulated the Galaxy for over 1,500 years. One of the wildest displays of Vitiate's power came when he took over the mind's of eight thousand Sith simultaneously and used their combined power to perform a ritual that ripped the Force from an entire planet, with Vitiate absorbing all of that energy himself in the most complex piece of Sith magick ever performed.
The Ones
The Belam spirits in Legends are probably the most powerful beings in the timeline, although they do not necessarily use the Force. The Gods of Mortis, however, are Force wielders more attuned to the Force than nearly any mortal could be.
The Son and Daughter are the embodiments of the dark side and the light side, respectively, while the Father maintains the balance, embodying neutrality between the two, controlling them, while overseeing the Galaxy. The Force came seamlessly to the deities, and like they are in canon, they are some of the most overpowered characters in Legends.
Abeloth
In the Legends timeline, the Gods of Mortis cannot be talked about without mentioning the Bringer of Chaos, the Beloved Queen of the Stars, former servant of the Gods, then known as the Mother, Abeloth.
As the Mother, she drank from the Font of Power then bathed in the Pool of Knowledge, becoming Abeloth, a kind of Force demigod, becoming immortal. She was able to dominate the Son and Daughter, with the two only getting saved by the Father, with it taking all three Gods to merely trap her. She could possess multiple people at once and could resist any attack and reform after any damage done to her, and she was also capable of creating her own warped reality. Luke Skywalker himself said Abeloth was beyond human comprehension. She was defeated but never really killed and remains the Legends timeline's most powerful character.
Luke Skywalker
The most powerful mortal in the Legends timeline, surpassing a long line of Sith and Jedi, and only being surpassed in powers by beings who transcend mortality and even the Force, is undoubtedly Luke Skywalker.
Luke very well may be the most powerful Jedi in canon also, but that has not been explored anywhere near as much as it was in Legends, which plays into why he was so overpowered. Luke had complete control of the Force and an immense understanding of it, achieving oneness at one point, which is a complete emergence with the Force, becoming one with it while still being alive, surrendering completely to the will of the Force. There are so many examples of his power being so ridiculously over the top; perhaps the wildest one that sums it up best is that he actually used the Force to manipulate black holes, an incomprehensible ability.