What a Knightmare!
One of the most expensive cards of all time, this card made Kozmo a tier 1 deck after its printing in Dimension of Chaos.
Kozmo Dark Destroyer
Thunder Dragon Colossus was often called this floodgate on legs.
Mistake
Dragunity
Dragunity Divine Lance
Ironically, this extra deck monster is rarely summoned, and is typically sent for an End Phase search for a negate or follow up.
Titaniklad the Ash Dragon
Moulinglacia the Elemental Lord (Battle Phase)
This card was played by Bandit Keith, representing the true American dream of gambling on Twitch.
Slot Machine
This card shuts down the activation of other field spells and was ironically played in Mystic Mine.
Field Barrier
Spellbook
Spellbook of Life
This pseudo-Kaiju makes up for him outing your boss monster with a present!
Santa Claws
This card allows for a quick win in time, as long as you don't mind not having a draw phase.
Fire Cracker (Draw Phase)
This extra-deck card saw an errata due to its burn effect being non-once per turn.
Dark Strike Fighter
Put on the limited list, this card's intended defensive effect wasn't explored too much, but its abuse in FTKs was much more noteworthy.
Wall of Revealing Light
Because of FTK shenanigans and self-looping, this fire (for some reason) beast-warrior was semi-limited.
Manticore of Darkness
This old yet simple card is one of the many cards to have had a censored artwork re-released as a Lost Art.
Offerings to the Doomed (Draw Phase)
This card saw heavy usage as a side-deck tech against Drytron as a target to add off of Orcust Crescendo.
Ally of Justice Cycle Reader
This card saw niche play versus combo decks as a way to pseudo mystic-mine level 6 or higher monsters.
Grave of the Super Ancient Organism
Karakuri
Karakuri Gama Oil
Ruling shenanigans and poor card text caused this monster to be enough of a nightmare that it would be limited until its errata.
Twin-Headed Behemoth
Played in Danger Dark-World FTK, this monster let you dig JUST a bit deeper for 500 life points when summoned.
Dr. Frankenderp (Draw Phase)
This monster saw early competitive play in Extra Deck Monarchs thanks to its level and ability to search, but was quickly revealed to be overhyped.
Sphere of Chaos
What we're all calling on.
Discord
Jinzo
Amplifier
Played in an infamous stun deck last year, this card allowed for easy token generation for both you and your opponent.
Dragonoid Generator
Ideally, you'll usually want to be destroying your own backrow with this card, as it ideally will be able to trigger their beneficial effects, such as summoning free bodies as well as destruction effects.
Artifact Ignition (Battle Phase)