Here is a list of all games I have a mastery badge on Retro Achievements. A mastery badge is a profile badge showing that all of the achievements in a game have been, well, achieved. All of these are done in "Hardcore Mode" which means no use of save states or rewind or other such emulator features. I don't have a whole lot of games on this list that were very difficult, but that is because most difficult games are insane challenges that I don't generally have the time to play a single game enough to get. I am going to try and hit some more difficult ones, but it won't probably happen for a while.
Games not on Giant Bomb:
Pringles Games (Genesis)
SNES Burn-in Test Cartridge
Diablo (Game Boy Prototype)
NES Control Deck Test Cartridge
Pocket Meat (GBA Meat Boy homebrew)
Dice of Date (Arduboy)
Ardu-EZ Button (Arduboy)
Bomber (Brazilian Bomberman clone, Mega Drive)
It's Corn! (WASM-4)
Texas Chainsaw Dodge (WASM-4)
Tetris26 (Atari 2600 homebrew)
Totally Not Sumo (WASM-4)
WLOKU (WASM-4)
Pong (WASM-4)
Minigolf (WASM-4)
A 20 Second Platformer (Game Boy Color homebrew)
Flappy (Atari 2600 homebrew)
Rick Astley: The Game (Game Boy Color homebrew)
Mind Teazzer (3DO)
Batman (SNES Software Creations Prototype)
Rayman (SNES Prototype)
Untitled Racing Game (SNES Prototype)
Big Burger (Nintendo 64 Prototype)
Touhou (Atari 2600 Homebrew Demo)
First Flight (WASM-4)
WarioWare, Inc. Trial Version (GBA)
Holidate (Arduboy)
Fluffles Rainy Day (Arduboy)
Petals Around the Rose (Pokemon Mini, Homebrew)
Pokemon Orange (Pokemon Mini, Homebrew Demo)
P-Type (Pokemon Mini, Homebrew Demo)
Sonic Arena (Pokemon Mini, Homebrew)
Zelda Mini (Pokemon Mini, Homebrew Demo)
Hamburger in Space! (Pokemon Mini, Homebrew)
gjpg (Pokemon Mini, Homebrew Demo)
Persona 4 (Game Boy, Homebrew Demo)
Wall-E (Atari 2600 Homebrew)
Katamari (Atari 2600 Homebrew)
Snake (Atari 2600 Homebrew)
Kung-Fu Street (Watara Supervision)
Rick and Morty Game (Arduboy)
Galactea (Arduboy)
Tiny DDug (Arduboy)
Kemono Patrol (Arduboy)
Hangman (Arduboy)
Rhythum (Arduboy)
Blackjack (Arduboy)
Among Us (Game Boy Homebrew)
Mega Man (Atari 2600 Homebrew)
THE Best Text Adventure (Game Boy Color Homebrew)
Diamond Run (NeoGeo Pocket Color Homebrew)
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1. Felix the CatFelix the Cathttps://giantbomb.com/wiki/Games/Felix_the_CatOpens wiki page
This is for the Game Boy version. I originally mastered it as my first game, but more achievements were later added. I went ahead and re-mastered it acquiring the new achievements.
2. Venetian BlindsVenetian Blindshttps://giantbomb.com/wiki/Games/Venetian_BlindsOpens wiki page
Very easy master for an incredibly simple "game". Basically a joke achievement set that involves a lot of raising and lowering blinds.
3. Taboo: The Sixth SenseTaboo: The Sixth Sensehttps://giantbomb.com/wiki/Games/Taboo_The_Sixth_SenseOpens wiki page
This "game" was really not very fun to master. It involved lots of fast forward and hoping for a lucky run through.
4. The Great Waldo SearchThe Great Waldo Searchhttps://giantbomb.com/wiki/Games/The_Great_Waldo_SearchOpens wiki page
NES Version.
5. SpongeBob SquarePants: SuperSpongeSpongeBob SquarePants: SuperSpongehttps://giantbomb.com/wiki/Games/SpongeBob_SquarePants_SuperSpongeOpens wiki page
GBA Version.
6. Cookie ClickerCookie Clickerhttps://giantbomb.com/wiki/Games/Cookie_ClickerOpens wiki page
This is actually an NES homebrew version.
7. Thomas the Tank Engine & FriendsThomas the Tank Engine & Friendshttps://giantbomb.com/wiki/Games/Thomas_the_Tank_Engine_And_FriendsOpens wiki page