I am of the opinion that the Yu-Gi-Oh OCG and TCG have 4 generations.
- Early Yugioh
- Later Yugioh
Early Yu-Gi-Oh! (Generation I)
This generation of Yu-Gi-Oh! TCG and OCG consists of anything and everything from Legend of Blue Eyes White Dragon until Labyrinth of Nightmare. I count these 5 packs as their own generation because they mainly contain weaker monsters with no effects, which although still exist in newer packs, are much fewer in numbers. I consider Labyrinth of Nightmare to be the final pack of Generation I because it contains the first Beatdown monster, Gemini Elf. This card has become symbolic of beatdown monsters. Also, this is the final pack to only include a handful of Effect monsters.
Later Yu-Gi-Oh! (Generation II)
This generation of Yu-Gi-Oh! consists of Legacy of Darkness until Flaming Eternity. I consider this a separate generation from the prior packs because it contains over 30 Effect monsters, 3 Beatdown monsters, and a new subtype, Spirits. Although Archtypes existed prior to this in the form of the Gravekeeper cards, this generation gave birth to a whole new breed of cards, making a majority of the cards from Generation I completely obsolete. The only cards that were not obsoleted by Generation II were set to become classics, such as Dark Magician, Blue Eyes White Dragon, and Red Eyes B. Dragon. This generation also included the introduction of LV monsters, such as the Armed Dragons and Horus, which have skyrocketed in usage since their introductions.
Yu-Gi-Oh! GX (Generation III)
This generation of Yu-Gi-Oh! is very obvious for its split from Generation II. From The Lost Millennium until its final pack, Light of Destruction, Generation III contained was a complete overhaul of metagame. From the introduction of the ever popular Elemental Heroes in the Lost Millennium to the introduction of the instant classics, the Gladiator Beasts in Gladiator's Assault, the idea of an archetype based deck, which was impossible prior to this point, is now more than possible. Almost every highly placing tournament deck was based around some sort of archtype.
From The Duelist Genesis and on until the present, this generation contains the first new type ever, the introduction of the Synchro Monster, which can be considered a superseding of Contact Fusion, and more archetypes.