Where it all started — 151 Pokémon, 8 gyms, one Rhydon prototype.
Red, Blue, Yellow (and the FireRed/LeafGreen remakes) · Released 1996
Kanto is the original Pokémon region, introduced in Red & Blue in 1996. The 151 Pokémon of Gen 1 defined the franchise's visual language — Ken Sugimori's hand-drawn art style, the first-generation type chart without Dark or Steel, and the 'starter trio' formula (Grass / Fire / Water) that every generation since has followed. Every competitive Pokémon metagame, every trading card set, every mainline game traces back to this one region.
Kanto is modelled on the real Kantō region of Japan, with Pallet Town standing in for rural Tochigi and Celadon City for Shinjuku. The original games had no secondary types for most Pokémon, no Special Attack / Special Defence split, and no abilities — these systems were all added in later generations and retroactively applied to Kanto mons in remakes.
Mew was added to the Red & Blue game data by developer Shigeki Morimoto in the last week of development as an Easter egg — only possible to obtain legitimately via the Nintendo Space World tournaments until the Gen 3 remakes made it accessible in normal play.
Red and Blue shipped with a single Special stat governing both special attack and special defence. That one decision broke the balance of the entire game: a Pokémon with high Special was simultaneously the best special attacker and the best special wall, which is why Mewtwo and Alakazam were untouchable and why the Psychic type had almost no counterplay — Bug was its only real weakness and Gen 1's Bug moves were feeble.
There were no abilities, no held items, no breeding and no natures. Critical hits were tied to a Pokémon's base Speed rather than a flat rate, meaning fast Pokémon crit far more often. A long list of moves simply did not work as described, and the infamous 1-in-256 accuracy failure meant even a 100%-accuracy move occasionally missed.
Almost everything that makes modern Pokémon a deep strategy game was added later. Playing Red and Blue today is worth doing for the history, but the systems you learn there will not transfer cleanly to any generation after the second.
Kanto's roster has aged better than its mechanics, largely because later generations kept coming back to it. Gen 2 added Steelix, Scizor and Blissey as evolutions of Gen 1 species; Gen 4 added Magnezone, Rhyperior and Electivire; Gen 6 handed Charizard, Mewtwo and Gengar Mega forms; and Gen 7 gave nineteen Kanto Pokémon Alolan variants.
The practical effect is that a Gen 1 Pokémon is often only competitively viable through a later addition. Base Scyther is unremarkable; Scizor was a top-tier threat for a decade. Base Onix is one of the weakest fully-evolved Pokémon in the game; Steelix is a genuine wall.
151, numbered #001 Bulbasaur through #151 Mew. That figure has stayed fixed since 1996 — Kanto is the only region whose Pokédex has never been expanded, though many of its Pokémon later gained new evolutions and regional variants that are numbered elsewhere.
Because Special Attack and Special Defence were a single stat, so Psychic types were the best special attackers and the best special walls at the same time. Their only listed weakness was Bug, and Gen 1's Bug moves were too weak to threaten them. The Dark type, introduced in Generation 2, exists largely to fix this.
Several are, but usually through later additions rather than their original forms — Scizor, Steelix, Blissey and Magnezone are all Gen 2 or Gen 4 evolutions of Gen 1 species, and nineteen Kanto Pokémon received Alolan variants in Generation 7.