Sauron

The big man himself.

Sauron (aka Mairon, Annatar, Sauron the Great, Gorthaur the Cruel, Sauron the Fair, The Enemy, The Dark Lord, the Great Eye, and Kathleen Kennedy) is the Big Bad Evil Guy and titular character of the Lord of the Rings franchise and the reason behind much of the changes and upheaval that the LoR world has faced during it's current ages. He is one of the first proper Dark Lords in popular fiction; while Darth Sidious, Doctor Doom and Abaddon were in kindergarten like some evil version of the muppet babies learning what the word 'bad' meant, Sauron was out cracking heads and getting shit done.

He wasn't the original big bad for LoR, oh no

A hilarious note is the fact he wasn't even the original big bad to begin with. Originally Sauron was known as Mairon (Quenya: Admirable) and was a second tier Maia under Aulë (the Valar of smithing and crafting) and he learned a lot from his old boss. A fundamental part of his mindset was a love of order, perfectionism and a hatred of waste and inefficiency.

Morgoth the first Dark Lord Giver of Freedom seduced Mairon into his new lifestyle by offering him unlimited chocolate cake forever and Sauron agreed, quickly becoming his chef chief underling. This was quite perplexing to those who originally knew Mairon, as he was a lover of order and organization where Melkor just liked to fuck shit up. But what actually seduced Mairon into Melkor's service was that Melkor acted where the other Valar basically did jack shit. Despite being Second Banana to Melkor, Sauron was somewhat less evil. Melkor's end game was to destroy the world and recreate it from the ground up, Sauron by contrast lacked his master's omnicidal vision. He was fine with the world, so long as he could rule it and structure it as he saw fit; and he saw being right hand to Melkor as a prime opportunity to do so once he was done fucking up the existing one.

Queue a long string of fucking shit up along with his new boss, until he ran into a girl and her dog (read: an OP elven princess and the hound of the local equivalent of the god of the hunt), and got himself almost killed. Once his boss was beaten up by the other Ainur (Tolkien's versions of archangels in this universe), Sauron fled across the sea to Middle-Earth, where he used his great cunning and sorcery to work events to his favour.

Damn those pesky kids!

Sauron covers all the angles.

So, with every Valar, Maiar and everything in-between either back to Valinor or hiding as hard as they could just in case they decided to come back, Sauron was certain it was now his time to shine. Still, after seeing how 'fuck things up' did Morgoth in, he went for the subtle approach. Taking a fair appearance and calling himself Annatar (Bringer of Gifts), he tried to corrupt the Elves by 'helping' develop their magic. The Elves, however, weren't all complete dumbasses and while they listened to him and eventually started forging Rings of Power like he was showing them (and giving those to the Men and Dwarves to keep up with his new name), they didn't trust him. So when he enacted his first 'Super Duper Master Plan to Conquer the World' and forged a Ring of his own, that was supposed to let him control all the other Rings. Except it didn't work! The Elves flipped him the bird and hid their Rings, merely having to be very careful when using them lest attracting his attention. The Dwarves too proved resistent to his power and did not fall under his thrall, albeit they became clannish and insular as a result. Even with Men it didn't work as intended: sure, the nine human kings became the Nazgûl, mightiest of his slaves... but the other humans would never follow them. Queue him flipping his shit, starting a good ole war to conquer everyone forcefully, and everyone uniting against him to kick his ass. Which happenend when the Numenorians (the LotR equivalent of Atlantis) joined the dance.

With his army beaten, Sauron decided to try subtlety again: he surrendered to the Numenorians and started corrupting them from within. Eventually, Sauron convinced them to attack the Valar by sea to steal immortality from them. While this might not sound like much, it is pretty much the equivalent of mortals trying to invase Paradise/the Garden of Eden by force (to keep with the Judeo-Christian parallels). Eru (the LotR equivalent of the Judeo-Christian God) was a little miffed by this, so he intervened directly to destroy the attackers, sink Numenor, force Sauron back to Middle-Earth in tatters; and ultimately completely separate the Undying Lands from Middle-Earth for good measure while muttering "get off my lawn!". Do note that Sauron surpassed Morgoth in this, for Eru never intervened directly against him. Yes, God with a capital 'G' had to personally give him the boot (though this is partly because the Valar by this time were very reluctant to intervene at all, having effectively nuked Middle Earth twice already to stop Morgoth); that must count for something on the 'evil villain' scale.

After that highest/lowest point in his career, Sauron, like any true evil villain was then beaten time and time again but he always managed to flee while screaming "I'll be back" over his shoulder. Finally having had enough of subtlety, he tried again to beat everyone to death in his big destroyer form with an army of orcs but thanks to some pesky men and elves, he got his finger with his magic ring (which contained a great deal of his power and life) cut off and as a result became a highly pissed off spirit that kept trying to come back, knowing that if he could get his bling back again he could have another bash at taking over the world.

Rise and fall again, this time for good

Sauron managed to crawl (do spirits crawl?) his way back from the abyss through sheer stubbornness and over time rebuilt his strength. When he felt secure enough he returned to his dark kingdom Mordor and began preparing for his final war against the kingdoms of men and elf, breeding an army of Uruk-Hai (think Primaris Marines with a whole lot more evil) to do his bidding. But you remember that bling we mentioned earlier? Some dang pesky hobbit kid only went and dropped it in Mount Doom! The one volcano that could destroy it! You know what we are saying?! Yep, it was bye bye for good this time Sauron and the one guy who made the world interesting popped his cogs.

And everyone went home in time for tea elevensies.

If you want to watch him work

Of course, there's the obvious prologue scene in the first LOTR film, but you can also have him as a hero unit in the Battle for Middle-Earth 2 videogame, if you play any of the 3 Evil factions. Be warned, you have to find and kill Gollum (and he is a BITCH to find) just to get his bling to be able to summon him. Then you have to pay a LARGE amount of your resources and wait for him to turn up. But seriously, for a guy who can smash apart entire squads IN ONE HIT, call down a hail of molten rock and scare anything attacking him into running away? He's worth it. Every second you wait to get him, he will pay back in full. (Just don't drop him in the middle of the enemy, dumbass.) And the best bit? All that shit is fucking CANON.

Will the real Slim Sauron please stand up

And then came the Amazon TV Series 'The Rings of Power' who cast an Eminem impersonator to be their Sauron. Immediately sparking mockery across the internet. For those too busy to read the Silmarillion article, Sauron never disguised himself as a human named "Halbrand" wearing cheap hot topic clothing, nor did he have anything to do with Galadriel; in all likelihood she would've been among the few elves who could've figured out his true identity, even in disguise.

Of course the writers were idiots and as such made her a complete dumbass who couldn't figure this out until he literally told her. Oh, and her rejecting his romantic advances ends up causing him to go into Mordor and he prepares to forge the One Ring. So he becomes the Sauron we all know and love because his crush told him to fuck off.

In other media

Sauron, being one of the first Dark Lord type characters in modern fiction, has made appearances, been referenced, and had homages to in various other forms of media. This includes being the namesake of the "Sons of Sauron" biker gang of Orks & Trolls in Shadowrun who believe intense acts of physical violence the only way to stop the likes of Humanis Policlub and other racist groups hellbent on the extinction of Orks, Trolls, and to a lesser degree other demihumans. There was also a /v/idya series called "Overlord" which basically allowed you to play as an expy of Sauron, controlling a horde of Goblins and slaying parodies of the Fellowship of the Ring who were each corrupted into an aspect of the 7 Deadly Sins by your predecessor, who was possessing the body of a Gandalf expy.