Illegal

If a card is Illegal, it means it cannot be used in the official card game at all, not in tournaments, not anywhere. The only exception to this is friendly duels where both duelists agree on the illegal cards in question in terms of their effects. So far, these are the only illegal cards:

English

 * Armament of the Lethal Lords
 * Chimaera, the Master of Beasts
 * Emperor of Lightning
 * Get Your Game On! (Only in Official Duels)
 * King of Destruction - Xexex
 * Meteo the Matchless
 * Obelisk the Tormentor
 * Slifer the Sky Dragon
 * Queen of Fate - Eternia
 * Testament of the Arcane Lords
 * The Seal of Orichalcos
 * The Winged Dragon of Ra
 * Ulevo

Asian-English

 * Firewing Pegasus
 * Kanan the Swordmistress
 * Obelisk the Tormentor
 * Slifer the Sky Dragon
 * The Winged Dragon of Ra

Japanese

 * Black Luster Soldier (DM)
 * Fiend's Mirror (DM)
 * Obelisk the Tormentor
 * Masahiro the Dark Clown
 * Slifer the Sky Dragon
 * Super War-Lion (DM)
 * The Winged Dragon of Ra
 * Tomohiro the Mountain Warrior
 * Yasushi the Skull Knight
 * Zera the Mant (DM)

Korean

 * Obelisk the Tormentor
 * Slifer the Sky Dragon
 * The Winged Dragon of Ra

It should also be noted that all fan-made and counterfeit cards are also illegal.