Armor of Invincibility

The Ultimate Blacksmith sends you out to hunt monsters because he wants to forge you something good but needs rare materials randomly dropped from enemies located around the world. And when you bring them back he forges you...a shield? What about the Infinity+1 Sword, you wanted a sword! In the Very Definitely Final Dungeon, you just beat That One Boss and open the chest behind it to find...a suit of Diamond armor, what the hell? Well you put so much work into finding it, may as well equip it. So you go into the menu and highlight this new item and...oh. Oh!

I'm sorry, did you just say the armor is made of diamonds or WIN!?

You just won the Armor of Invincibility, the best defensive item in the game, bar none, with stats far surpassing the other equipment pieces you can expect to find, and may offer protection from Standard Status Effects and Elemental Powers. Demonic Spiders don't even dent it, and even the Boss Battles have become Curb Stomp Battles with it on. This is the shield to go in your other hand while you grasp the Infinity+1 Sword in the other. As with the Infinity Plus One Sword, the only real drawback is that you'll be required to sink hours into finding the Armor of Invincibility, maybe even doing That One Sidequest. May just be a Bragging Rights Reward. Or, if you plan to face the Bonus Boss, it may be a requirement to avoid getting killed in one hit.

Video Games

 * Castlevania: Circle of the Moon has two examples. The first being the "Shinning Armor" which boosts all your stats in addition to its outstanding defense. To get it, you have to beat The Arena, a gauntlet filled to the brim with Demonic Spiders that you cannot use magic in. Its counterpart, the Dark Armor, boosts defense even more, but actually cuts the rest of your stats. It's also simpler to acquire, being a simple drop (albeit one from a somewhat rare enemy).
 * The Elder Scrolls Four, the player can enchant pieces of clothing with resist damage effects or resist elemental damage effects that resist both damage from the elemental effect and normal damage. One can enchant clothing with chameleon effects which cause partial invisibility. A character can wear gauntlets, boots, greaves, a cuirass, and a helmet or hood as well as a necklace and two rings. Enchanting enough armor or clothing with damage resistances can result in a suit of 85% damage resistant armor. True invincibility can be attained by enchanting enough pieces with chameleon which will make the player totally invisible.
 * Or 100% Reflect Damage, which nullifies every attack that is done to you, and reverses it on the attacker. Only way to die now is via magic or arrows. And you can very easily make a custom enchantment that reflects all magic at the caster. So... all that's left are arrows, which, by the end of the game, can't hurt you. At all.
 * Unless, of course, they pierce you in the patella.
 * Final Fantasy IV, the Adamant Armor has 100 defense, resists almost all status ailments and elemental damage, and gives a big boosts to stats. Finding it requires you to find the Pink Puff randomly in a single room in the final dungeon, win the Pink Tail from it which drops very rarely (combined with the puff's rarity you have less than a 1/10000 chance to get it) and then trade it for the armor.
 * The Advance release introduced the Hero's Shield, the spiritual sucessor to the Paladin's Shield (see below) which is won from a Bonus Boss in the Bonus Dungeon, while the the DS release introduced a whole subset of this called Onion Equipment. Like the Adamant Armor you need to collect tails randomly dropped from enemies.
 * Final Fantasy VI has the Paladin Shield, which has 59 defense and magic defense, boosts evade and magic evade by 40%, and nullifies or absorbs all elemental damage. Not only that, but it is also the only item in the game aside from the Ragnarok magicite that teaches you the Ultima spell. To get it though you need to fight 256 battles equipped with the Cursed Shield, which inflicts all status ailments on the wearer.
 * With a defense rating of 128, the Snow Scarf gets you more than halfway to the max possible defense rating, and with a decent helmet and shield most attacks will only do 1 damage to you.
 * Final Fantasy VII gives us the Ziedrich, which has 100 defense, halves all elemental and physical damage, boosts your stats and can only be stolen from three bosses (well, technically, the same boss three different times), one of which can be missed entirely. The catch is that it has no materia slots, making it of somewhat dubious value.
 * There's also the Mystile. While it doesn't gives the highest defense, it has huge evasion bonuses and 3 pairs of materia slots. You can only find it in 2 chests, but there's a Good Bad Bug that allows you to get a third.
 * Final Fantasy IX has the Tin Armor, which can only be acquired by defeating a Bonus Boss and having him synthesize it from two items, one of which is incredibly common and the other of which can only be acquired by doing a lengthy sidequest.
 * The Armor of Invincibility in Final Fantasy XII is exclusive to the Japanese-exclusive re-release, the Gendarme. It boosts evade and magic evade by 90 and resists all forms of elemental damage.
 * In most given Kingdom Hearts games, odds are that the Ribbon will be the best equipment available. It usually has the best defensive boost of any armor and also gives resistance to all elemental damage.
 * In the Fallout series, odds are that the Armor of Invincibility is some kind of Power Armor, though the exact make and model vary from game to game.
 * Super Mario RPG has the Lazy Shell. It does do terrible things to all the non-defensive stats, but that's not really a problem if you give it to Peach, who wasn't going to do anything but heal, anyway.
 * Pokémon has the infamous "Curselax" set in its Gameboy Colour and Gameboy Advance releases, which lowers your speed (which Snorlax doesn't care about in the first place) tremendously, but improves Snorlax's mediocre defense, which only aids his phenomenal special defense. It doesn't hurt that it raises it's attack too, making it something of a Game Breaker in those generations.
 * Many moves raise defenses, including Cosmic Power, Quiver Dance and Bulk Up to name a few. The stand-out for pure defense though is Cotton Guard, which grants +3 stages of increase to defense, when the max increase is +6 stages of increase. Each stage raises defense by 50%, and stacks additively, meaning you'll have tripled your defense after two uses of the move.
 * Star Ocean the Second Story has the Bloody Armor, which has seemingly low stats and constantly drains your health at an astounding rate, but wearing it makes you immune to any and all damage and keeping up with the constant HP drain isn't that much of an issue with an assigned healer.
 * The Fusion Helmet in Shadow Complex. You're required to scour the map for all the passkeys, but if you get it, you take little damage from most attacks. Better yet, if you are completely still or moving VERY slowly, you gain a forcefield, making you pretty much completely immune to every attack in the game.
 * The Body Armor in Vampire: The Masquerade Bloodlines. It is only obtainable in the endgame sequence, and even then only at one point in-between missions. The actual acquiring is quite simple though, as you only need to talk to your ghoul to get it. If you do get the body armor, you'll be pleasantly surprised to find it gives 4 damage soak to bashing and lethal, render you nearly immune to most melee weapons or small arms fire the game will throw at you. However, it does nothing for the aggravated damage dealt by many of the enemies you'll meet in melee during the endgame.
 * Minecraft has armor crafted from diamonds. However, you'll need a lot of diamonds; even basic boots will take four diamonds to make, let alone breastplates or pants.
 * The 1.0 release had a bug where damage reduction from armour was applied twice, making a player wearing a full suit of diamond practically invincible. This greatly impacted on the strategies employed during the first reddit "race for wool" tournament as obtaining diamond armour or denying it to the other team would often decide the match.
 * Dragon Age: Origins The Shimmering Shield buff grants a frankly preposterous armor bonus of 15 (a full set of top-tier massive armor gives 32), maxes out all elemental resistances, and gives gigantic bonuses to mental and physical resistance checks. It's Shimmering Shield that makes mages the best tanks in the whole game.
 * Skyward Sword has the Hylian shield, which is protected from both fire and electric attacks, and is the only shield in the game that won't break if it takes too much damage.
 * Wind Waker also had a set of magic armor that rendered Link invincible, at the cost of constantly draining his magic for as long as it was active.
 * All three of the Assassin's Creed games starring Ezio Auditore features a set of special armour that gives you the highest HP rating in the game and is also completely unbreakable. In Assassin's Creed II and Assassin's Creed: Brotherhood, getting the Armours of Altair and Brutus took most of the game, but in Assassin's Creed: Revelations, it's possible to gain one of TWO such suits by the second memory sequence, making it a veritable Disc One Nuke.

Web Comics

 * In Eight Bit Theater, obtaining the Armor Of Invincibility was Fighter's original goal. He did obtain the Armoire of Invincibility, which proved equally invincible but significantly less useful.