The answer may surprise you.
What do you think is the hardest achievement of all time? Going into this video, before doing any research, I thought Seriously 3.0 from Gears 3 was the toughest. I am by no means an expert in the area of difficult achievements. The rarest achievement I have unlocked is Zealot from Halo: MCC, which you get by " Halo 2A MP: In Anniversary matchmaking, win 10 games as an Elite." Yeah, seriously, that's my rarest one. But I do have Overwatch's Rapid Discord unlocked, which would be my rarest achievement if it weren't for me playing Overwatch on PC (it's on Battle.net, not Steam). Now the hardest achievement I have unlocked is arguably beating CoD: World at War or CoD 4: Modern Warfare on veteran (I know there isn't one single achievement for doing this, but still). The two campaigns are about the same difficulty. So, in the grand scheme of things, I haven't unlocked any extremely difficult achievements, which is both a product of me not going for them and me not being good enough to get them without melting my brain. I had a ton of help from the fine users of True Achievements, who have made plenty of informative guides and comments. This video would look nothing like it does without True Achievements existing.

Music used in order of appearance:
Saki Kaskas – Callista
The Algorithm: Dynamic Recompilation
Super Meat Boy – Dr. Fetus’ Castle
Cod: Black Ops 3 - Ignition
Need for Speed – Rom di Prisco: Menu Theme
Cyber Marian - Piłko Moja Kochanka
Asteroids for Game Boy – Title
Peacemaker – Nero’s Law
The Elder Scrolls: Oblivion - Reign of the Septims
FM-84 - Tears
CoD: Modern Warfare 2 – Spetsnaz Theme
Star Wars Episode 1 - Augie's Great Municipal Band and End Credits
Battlefield 4 – A Theme for Kjell
Conquering Dystopia – Destroyer of Dreams

Video footage sources in order of appearance:
Ghost Recon: Advanced Warfighter – Menu Theme
Raining diamonds animation: xponentialdesign
Dead Rising 7 Day Survivor gameplay: Recommended Playing
Rock Band 2 Bladder of Steel gameplay: Chris265265
Cars 3: Driven to Win trophies: Poru99
Metal Gear Solid 4 Platinum trophy: cinnabon7354
Top 10 Rare Replay: Xbox On
The Simpsons Game start screen: ★WishingTikal★
Counting/falling numbers animation: footageisland
War of the Worlds gameplay: XBLA Fans
Grand Theft Auto V gameplay: drkj
Outlast 2 gameplay: IFreeMz
Garry’s Mod gameplay: SpyCakes
The Stanley Parable gameplay: Xanifur
Black Ops 3 campaign gameplay: NoughtPointFourLIVE
Black Ops 3 medals showcase: Indecisive Prophecy
Need for Speed Prestige speed events: VGSpeedPro
Pure Hold’em gameplay: Lil Draven
Pure Hold’em level up screen: INFINITE 1112
Pure Hold’em gameplay with text callouts: ICanNotFindAName
Vanquish gameplay: Dead End Kyo
Robotron: 2084 wave 100 gameplay: mecharichter
Robotron: 2084 low level gameplay: Delisted Games
Rare Replay intro and all stamps collected: BeatsByDay
Cloudberry Kingdom level 319 full speed gameplay: Jerrydactyl
Cloudberry Kingdom level 320 slow-mo gameplay: HiddenLongshot
Lost Planet 2 campaign gameplay: Levan
Lost Planet 2 multiplayer clip: kiranearitachi
Lost Planet 2 Queen boss fight: CaleoGaming
Elder Scrolls Online Elsweyr gameplay: Lazy 4K
Elder Scrolls Online former emperor: Gare
Elder Scrolls Online PVP Siege gameplay: Adrian Daniel
Deathsmlies level 999 playthrough (MBL arcade): Reazie
Tempest round 81 checkpoint and arcade gameplay: TenTokensForADollar
Tempest round 96+ gameplay: Old Classic Retro Gaming
Need for Speed Prestige drifting race: Reiji

All other gameplay is my own.

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