When the phrase "tinpot dictator" is literal.
Evil Overlords have the tendency to fight in full plate armor, even in settings where that doesn't actually make sense. Their armor usually has plenty of Spikes of Villainy and is Color-Coded for Your Convenience in black. Usually, it conceals their face to make them appear more mysterious and menacing. Often upgraded to Powered Armor in more advanced settings.
In fantasy settings, this is the standard look for the Black Knight. However, that doesn't always mean that an overlord of this type is solely a physical fighter. He might also be an Evil Sorcerer that made the wise choice of donning armor to offset his main weakness. Besides, they are almost always Large and in Charge so it's normal to see them as evil Magic Knights.
Spikes of Villainy and a Badass Cape are commonly used accessories.
See also Animated Armor.
Examples
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Anime & Manga
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◊.
Wapol from One Piece had a ship called "The Tin Tyrant" but Don Krieg fits the bill better albeit without a helmet.
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.
COBRA Commander, in the middle of the Marvel Comics run where he always wore his battle-armour. It had the side benefit of concealing the fact he was an imposter.
Films — Live-Action
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". Morgoth is described as wearing black armor in combat (though outside of it, at least his hands and face are left uncovered).
Sith in general are likely to be this; many of them were famous for their unique suits of armor.
Also every Mandalorian ever Mandalore, 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.
Literature
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.
Live-Action TV
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.
Tabletop Games
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 Eldar Archons usually wear heavy armor in the style of the Incubi, instead of the form-fitting constructions worn by most of their warriors.
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.
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.
Video Games
The Final Fantasy series' more medieval installments love this trope, as seen at the top of the page.
Garland in Final Fantasy I. (although his characterization in Dissidia is more similar to that of a Blood Knight)
Emperor Mateus Palamecia in Final Fantasy II, although he doesn't have a helmet, he certainly has a golden and violet suit of armor.
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.
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.
In his case it's justified. His body is slowly wasting away due to his vast power. The armor 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 battle with him. 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. His new Pentakill Mordekaiser skin removes the armor covering his torso, but he is still wearing his helm.
Lord Deimos from Mace: The Dark Age fits too, seeing as he's encased in blood red armor and we never see any part of him outside of it. He even has the BFS and Spikes of Villainy
Frank Horrigan from Fallout 2 is a 12-foot tall, Powered Armor version of this. The Roman-themed Legate Lanius from Fallout: New Vegas is the more traditional version.
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.
Knight-Commander Meredith in Dragon Age 2 has got normal-looking plate armor as well.
Saren in Mass Effect has creepy-looking armor that looks like it has Geth parts fused to it.
The Reapers are all gigantic spacecraft, and thus have got pretty tough black shells.
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.
Dark Lord Gaol in Kid Icarus: Uprising. It turns out that the armor is actually used to control Gaol, who was a captured spy and is actually a woman. After defeating her, she makes a Heel Face Turn and learns to control the armor herself.
Megabyte in ReBoot is a borderline example. His body is his full metal armor. Though on one occasion Dot calls him a "blue tin coward" when Megabyte didn't show up in a battle.
Megatron in Transformers, being the leader of an army of Killer Robots, qualifies. Scorponok even calls the Beast Wars incarnation one on one occasion.
