The Mega Royal Family
One of the most infamous cards of all time, this card drastically increased the power level and the price of playing Yugioh.
Dark Armed Dragon
One of the best monsters of its type, this card has been playable in several formats for the last decade, including some very recent ones. It was originally the cover card of a Structure Deck, but was also reprinted in a later Structure Deck.
Machina Fortress
These two cards, released in the same set, were the first ever berserkers in the TCG.
Dark Elf and Jirai Gumo
Mammoth Graveyard
Dinosaur
PE Minor if you pronounce this card's name wrong.
Armor Exe
If Tribute Summoned, this monster makes it very easy to OTK thanks to its ability to deal massive damage in multiple attacks. But the deck it belongs to will almost always Normal or Special Summon it by other means.
Qliphort Shell
This was the first Link-4 monster in the TCG to debut in a Structure Deck.
Topologic Zeroboros
You need to get a German copy of this card so your opponent doesn't know you're cheating when you search it with Reinforcement of the Army.
Evil HERO Adusted Gold
XX-Saber Gottoms
Beast-Warrior
One of the most degenerate commons of all time, this card has been banned for almost 2 decades.
Magical Scientist
This is the only Normal Monster with 2800 ATK and 1000 DEF
Metaphys Armed Dragon
The Structure Deck this card released in was legacy support for an iconic but very bad group of monsters. While it's an incredible starter for the deck it belongs to, it was more commonly played as part of a small package in Spright decks.
Dark Beckoning Beast
This is the only hand trap berserker.
Iron Dragon Tiamaton
Exodia Necross
Spellcaster
Players commonly refer to this monster with a nickname based on the color of its clothes.
Spellbook Magician of Prophecy
This awful card was much harder to obtain in the TCG than in the OCG. It is one of the only cards in the game with an effect that activates in a player's Draw Phase before they draw.
Diabolos, King of the Abyss
This card debuted in an OCG Structure Deck but was released as a Secret Rare in a TCG core set. It was played in early DAD Return decks to fix the number of DARKs in the GY.
Prometheus, King of the Shadows
There are 6 Level 4 or lower monsters with at least 2000 ATK that also have 2000 DEF. Of all those monsters, this is the only one that saw competitive play, and was well-known for having confusing text and being annoying to play against.
Inspector Boarder
Bat
Machine
First edition, original print copies of this card that have the art Konami intended are extremely valuable. This is due to a mistake on Konami's part, and their misguided attempt to fix it.
Dark Paladin
This monster is technically one of the most powerful floodgates in the game, but it's too difficult to summon to be competitive, even in Goat format, and is outclassed by a certain FIRE Dragon in the same archetype.
Silent Swordsman LV7
The character in this card's art also appears on a Link Monster. Both this card and that Link Monster have the same ATK, and allow you to search for your all-important Spell/Traps, but they belong in completely different decks.
Springans Kitt
This card was the first FIRE monster to have piercing.
Gaia Soul, the Combustible Collective
Fleur Synchron
Machine
In a scripted duel event, Kaiba claims he will defeat Yugi because he's learned how to Synchro Summon. Yugi responds by Xyz Summoning this monster.
Ebon High Material