Berserk is naturally filled with villians (and some heroes) in this style.
Probably the most notable one is the legendary High King Gaiseric, also known as the Skull King for his preferred helmet style. In an interesting subversion, if he and the Skull Knight are one and the same person as most fans believe, he became a good guy (relatively speaking) after becoming undead.
Master Hades of the Grimoire Heart guild from Fairy Tail.
Wapol from One Piece had a ship called "The Tin Tyrant" but Don Krieg fits the bill better albeit without a helmet.
Yuki Judai of Yu-Gi-Oh! GX fits this trope in full, as his standard clothing during the Genocidal Overlord phase becomes a black, full-body armor, heavily accessorized with sharp spikes, jewels, a flowing red cape and a helmet which fully covers his face when the visor is lowered — as can be seen here◊.
Darth Krayt from Star Wars: Legacy is a bit of a subversion as the armor is a part of him it's Yuuzhan Vong armor.
To clarify, it is A collection of crustaceans that have bonded to his skin to provide lightsaber-proof armor at the expense of unending agony as they attempt to digest him. Darth Bane wore similar armor, but not of Vong origin..
The Lord Marshal of the Necromongers in The Chronicles of Riddick is rather nostalgic for a space conqueror, forgoing any sort of conventional or futuristic outfit for a plate armor suit that he never takes off, only removing his helmet from time to time. Most of his followers are no different, presumably because of their martial society.
Sauron, but he is only described in armor in the movies; in the book, he's simply described as "A Terrible Dark Lord", "an image of malice and hatred made visible". His master Morgoth is described as wearing black armor in combat (though outside of it, at least his hands and face are left uncovered).
The Witch-King is something in between this and Animated Armor, since he's a disembodied undead spirit and needs the armor as a surrogate body, to manipulate objects.
From Star Wars, the Sith in general are likely to be this; many of them were famous for their unique suits of armor. Darth Vader is probably the most obvious example, if not the Trope Codifier: he was most likely inspired by the Witch King of Angmar and Sauron, Doctor Doom, and armoured samurai from Japanese films, but he is probably the most famous example in popular culture. Vader's armor also doubles as a life support system, stemming from injuries he sustained in an old battle with Obi-Wan.
Mandalore, leader of the Mandalorians, in pretty much any era (although they also qualify as Blood Knights, seeing how they take joy in battle). Yuuzhan Vong love their living crab-armor, too, although the Supreme Overlord doesn't bother with it.
Being set in a medieval society, most of the lords in A Song of Ice and Fire wear heavy and elaborate plate armor when they go into battle. The Boltons' may be the most sinister, as its designed to look like a flayed man. And they are bastards.
In the original Dragon Lance trilogy, all of the Dragon Highlords had ceremonial plate armor with elaborate helmets. Verminaard actually fought in his. It backfired, as he had no peripheral vision in the armor, and when he suddenly lost his cleric powers, he was killed shortly afterwards.
In The Silmarillion, Morgoth takes to dressing like this after fleeing Valinor with the Silmarils.
Sauron is usually portrayed like that too by the artists.
Obould Many-Arrows from the Forgotten Realms books wears a spiked suit of black plate armor that's nearly impenetrable. They even used a spell to give him a transparent visor on his helmet so he wouldn't have vulnerable eye holes. Though, when he took the armor off, he got really dangerous.
The "evil" emperor in Journey of the Catechist wears full plate armor. Ironically, not only is he a powerful sorcerer who doesn't need it for protection, its used entirely to hide his appearance, which he considers ugly.
In the first few seasons of Stargate SG-1, Apophis wore the same armor his Jaffa had, only golden. In later seasons he eschews in favors of robes.
Chaos Lords from Warhammer Fantasy usually wear heavy plate armour decorated with loads of spikes and symbols of Chaos. Chaos Lords and Sorcerers from Warhammer 40,000 wear Powered Armor (or even heavier Terminator armour) decorated with loads of spikes and symbols of Chaos, as well as... er... trophies.
In fact at one point the mark of a true Chaos Warrior was being granted Chaos Armor, which fused to their skin, could never be removed, and if damaged would grow back.
Ork Warlords have a habit of turning up wearing mega armour, which makes them Rusty Iron, Tin, Ceramite, Adamantium, And Rubber Tyrants.
Dark Elf Dreadlords usually wear heavy plate armor, and in some cases don't show their faces often.
The ruler of the dark elves Malekith has to wear armor all the time. After the Flame of Asuryan opinioned that he was unworthy of becoming the Phoenix King he's needed a suit of magical armor to give his ruined body strength.
The Armored Megalomaniac archetype in Mutants & Masterminds, as well as the somewhat more specific example of Evil!Daedalus from Anti-Earth in the Freedom City sample setting.
Exalted gives us the First and Forsaken Lion, an ancient, tyrannical ghost empowered by the dead makers of Creation to go out and kill everything. He's a master warlord and strategist in a gigantic suit of armor. Not that he has much choice, as he was welded into it as punishment for his first major screw up.
One of the signature Abyssals, Falling Tears Poet, is clad in heavy armour, wields a really big hammer, and is approximately the size of a bus. Bear in mind that one of the potential end arcs for the Abyssals is to overthrow and replace the Deathlords.
The unofficial Lord of the Rings tabletop game Ambarquenta encourages this by ruling that metal contains Morgothian element and thus gets in the way of casting good wizardry and good-ish sorcery, but does not hamper evil sorcery and necromancy in any way.
The Final Fantasy series' more medieval installments love this trope, as seen at the top of the page.
The fal'Cie play with this, since they appear to be made of tin.
The Garlean generals in Final Fantasy XIV all dress like this. Unlike a number of examples here, their armor tends to be functional on top of the form - Gaius van Baelsar, for example, seems to have a number of gizmos, like a gauntlet-gun, installed in his armor.
All of the eponymous Overlords in the Overlord series.
Ganondorf from The Legend of Zelda has been seen in armor quite a few times, but never covers his head.
Death's Hand in Jade Empire is the Emperor's right hand man, who had his departed soul bound to a black suit of armour. Master Li declares that "That is the armour of a man who knows no remorse, no pity." The irony is that said armour originally belonged to Master Li.
Magruder in Gun. The reason, kinda, that he's completely unharmed by any of your weapons and has to be taken down by other means.
Guilty Gear: Surprisingly, Justice is this as well, since the drama CDs state that she wears it to protect her frail figure. The hair on the armor, though, is her real hair.
It's Spiritual Successor, Blazblue follows a similar trend with Hakumen, who combines this with being a Samurai Robot Unlike Justice however, Hakumen's armor houses his soul rather than his body. His original body was too broken to be of any further use, hence the upgrade.
In the first game of Lightning Warrior Raidy, one of the bosses is this. This hides the fact that inside, it's a girl.
In the Warcraft series, Arthas fits this trope quite well.
Deathwing as well, despite being a dragon. Unlike the other dragons which run around basically naked, Deathwing has elementium plates grafted onto his body. His body is slowly wasting away due to his vast power, and the plating is basically holding his body together.
In Myth: The Fallen Lords, Balor wears armor in combat, which is probably all the time. Given the game's heavy Tolkien influences, this is pretty fitting.
Xemnas from Kingdom Hearts II gets this form during the first and third stages of the final battle with him. He spends both fights slouching in the control chair of a spaceship, using his powers to fight. It turns out to be the Keyblade Armour of Master Xehanort, his original original incarnation.
Mordekaiser from League of Legends rules over... something and is constantly seen wearing armor. It's strongly believed that whatever's under that suit, it isn't anything friendly. His (noncanon) Pentakill Mordekaiser skin removes the armor covering his torso, but he is still wearing his helm.
Legate Lanius: We shall see how brave you are when you are nailed to the walls of Hoover Dam, your body facing West so you may watch your world die.
Loghain in Dragon Age: Origins is arguably an aversion, while he does run around in full plate armor it appears to be standard, ordinary, off-the-shelf full plate and is silver instead of black. Of course the armor was taken from another Tin Tyrant who occupied his nation.
Knight-Commander Meredith in Dragon Age II has got normal-looking plate armor as well.
Tell the Demon Sovereign from Might and Magic: Dark Messiah that Sauron is suing.
The Black Knights from Dark Souls are completely covered in their huge, tough black armor and that's all that's left of them. Also Havel the Rock, though his armor is actually incredibly heavy carved rock.
Discussed in Schlock Mercenary, where the pun of this trope is brought to its extreme after the events of Credomar. LOTA, a robotic longshoreman built from the remains of a ruined tank, is naturally covered head to toe in heavy armor. LOTA is then voted in as sole ruler of the colony after the battle over food supplies.