Sea Route 21: 44 species in 8 games

Road 21: 44 species across 14 versions

Red and Green (Japan): 12 species
Species	Method	Levels	Chance	Game page
Pidgey	Walking in tall grass or a cave	21 to 23	30%	All places
Pidgeotto	Walking in tall grass or a cave	30 to 32	15%	All places
Rattata	Walking in tall grass or a cave	21 to 23	30%	All places
Raticate	Walking in tall grass or a cave	30	15%	All places
Poliwag	Fishing with a good rod	10	50%	All places
Tentacool	Surfing	5 to 40	100%	All places
Shellder	Fishing with a super rod	15	25%	All places
Tangela	Walking in tall grass or a cave	28 to 32	10%	All places
Horsea	Fishing with a super rod	15	25%	All places
Goldeen	Fishing with a good rod	10	50%	All places
Goldeen	Fishing with a super rod	15	25%	All places
Staryu	Fishing with a super rod	15	25%	All places
Magikarp	Fishing with an old rod	5	100%	All places

Red and Blue: 12 species
Species	Method	Levels	Chance	Game page
Pidgey	Walking in tall grass or a cave	21 to 23	30%	All places
Pidgeotto	Walking in tall grass or a cave	30 to 32	15%	All places
Rattata	Walking in tall grass or a cave	21 to 23	30%	All places
Raticate	Walking in tall grass or a cave	30	15%	All places
Poliwag	Fishing with a good rod	10	50%	All places
Tentacool	Surfing	5 to 40	100%	All places
Shellder	Fishing with a super rod	15	25%	All places
Tangela	Walking in tall grass or a cave	28 to 32	10%	All places
Horsea	Fishing with a super rod	15	25%	All places
Goldeen	Fishing with a good rod	10	50%	All places
Goldeen	Fishing with a super rod	15	25%	All places
Staryu	Fishing with a super rod	15	25%	All places
Magikarp	Fishing with an old rod	5	100%	All places

Yellow: 10 species
Species	Method	Levels	Chance	Game page
Pidgey	Walking in tall grass or a cave	11 to 17	55%	All places
Pidgeotto	Walking in tall grass or a cave	15 to 19	10%	All places
Rattata	Walking in tall grass or a cave	13 to 15	30%	All places
Raticate	Walking in tall grass or a cave	15	5%	All places
Poliwag	Fishing with a good rod	10	50%	All places
Tentacool	Fishing with a super rod	15 to 30	60%	All places
Tentacool	Surfing	5 to 40	100%	All places
Tentacruel	Fishing with a super rod	30	10%	All places
Goldeen	Fishing with a good rod	10	50%	All places
Staryu	Fishing with a super rod	20	30%	All places
Magikarp	Fishing with an old rod	5	100%	All places

Gold and Silver: 8 species
Species	Method	Levels	Chance	Game page
Tentacool	Fishing with a good rod	20	35%	All places
Tentacool	Fishing with an old rod	10	15%	All places
Tentacool	Surfing	30 to 39	90%	All places
Tentacruel	Fishing with a super rod	40	20%	All places
Tentacruel	Surfing	35 to 39	10%	All places
Shellder	Fishing with a good rod	20	10%	All places
Shellder	Fishing with a super rod	40	30%	All places
Tangela	Walking in tall grass or a cave	20 to 35	up to 95%	All places
Mr. Mime	Walking in tall grass or a cave	28 to 30	up to 10%	All places
Magikarp	Fishing with a good rod	20	35%	All places
Magikarp	Fishing with an old rod	10	85%	All places
Chinchou	Fishing with a good rod	20	20%	All places
Chinchou	Fishing with a super rod	40	40%	All places
Lanturn	Fishing with a super rod	40	10%	All places

Crystal: 10 species
Species	Method	Levels	Chance	Game page
Rattata	Walking in tall grass or a cave	25	up to 30%	All places
Raticate	Walking in tall grass or a cave	20	up to 10%	All places
Tentacool	Fishing with a good rod	20	35%	All places
Tentacool	Fishing with an old rod	10	15%	All places
Tentacool	Surfing	30 to 39	90%	All places
Tentacruel	Fishing with a super rod	40	20%	All places
Tentacruel	Surfing	35 to 39	10%	All places
Shellder	Fishing with a good rod	20	10%	All places
Shellder	Fishing with a super rod	40	30%	All places
Tangela	Walking in tall grass or a cave	28 to 35	up to 60%	All places
Mr. Mime	Walking in tall grass or a cave	28 to 30	up to 10%	All places
Magikarp	Fishing with a good rod	20	35%	All places
Magikarp	Fishing with an old rod	10	85%	All places
Chinchou	Fishing with a good rod	20	20%	All places
Chinchou	Fishing with a super rod	40	40%	All places
Lanturn	Fishing with a super rod	40	10%	All places

FireRed: 8 species
Species	Method	Levels	Chance	Game page
Psyduck	Fishing with a super rod	25 to 35	1%	All places
Tentacool	Surfing	5 to 40	100%	All places
Krabby	Fishing with a good rod	5 to 15	20%	All places
Tangela	Walking in tall grass or a cave	17 to 28	100%	All places
Horsea	Fishing with a good rod	5 to 15	60%	All places
Horsea	Fishing with a super rod	15 to 25	80%	All places
Seadra	Fishing with a super rod	25 to 35	4%	All places
Magikarp	Fishing with a good rod	5 to 15	20%	All places
Magikarp	Fishing with an old rod	5	100%	All places
Gyarados	Fishing with a super rod	15 to 25	15%	All places

LeafGreen: 8 species
Species	Method	Levels	Chance	Game page
Tentacool	Surfing	5 to 40	100%	All places
Slowpoke	Fishing with a super rod	25 to 35	1%	All places
Krabby	Fishing with a good rod	5 to 15	60%	All places
Krabby	Fishing with a super rod	15 to 25	80%	All places
Kingler	Fishing with a super rod	25 to 35	4%	All places
Tangela	Walking in tall grass or a cave	17 to 28	100%	All places
Horsea	Fishing with a good rod	5 to 15	20%	All places
Magikarp	Fishing with a good rod	5 to 15	20%	All places
Magikarp	Fishing with an old rod	5	100%	All places
Gyarados	Fishing with a super rod	15 to 25	15%	All places

HeartGold and SoulSilver: 15 species
Species	Method	Levels	Chance	Game page
Tentacool	Fishing with a good rod	20	30%	All places
Tentacool	Fishing with an old rod	10	15%	All places
Tentacool	Surfing	30 to 35	90%	All places
Tentacruel	Fishing with a super rod	40	20%	All places
Tentacruel	Surfing	35	10%	All places
Shellder	Fishing with a good rod	20	10%	All places
Shellder	Fishing with a super rod	40	30%	All places
Tangela	Headbutting trees	27 to 30	15%	All places
Tangela	Walking in tall grass or a cave	20 to 35	up to 50%	All places
Mr. Mime	Walking in tall grass or a cave	28 to 30	up to 5%	All places
Magikarp	Fishing with a good rod	20	40%	All places
Magikarp	Fishing with an old rod	10	85%	All places
Hoothoot	Headbutting trees	24 to 30	up to 65%	All places
Spinarak	Headbutting trees	24 to 26	30%	All places
Chinchou	Fishing with a good rod	20	20%	All places
Chinchou	Fishing with a super rod	40	40%	All places
Lanturn	Fishing with a super rod	40	10%	All places
Linoone	Walking in tall grass or a cave	35	20%	All places
Wurmple	Headbutting trees	24 to 30	up to 20%	All places
Whismur	Walking in tall grass or a cave	25 to 30	20%	All places
Bidoof	Walking in tall grass or a cave	35	20%	All places
Buizel	Walking in tall grass or a cave	25 to 30	20%	All places

Let's Go, Pikachu and Eevee: 20 species
Species	Method	Levels	Chance	Game page
Charizard	Rare spawn in the sky with lure or catch combo	3 to 56	100%	All places
Pidgey	Walking in the overworld	37 to 42	100%	All places
Pidgey	Flying pokemon in the sky	3 to 56	100%	All places
Pidgeotto	Walking in the overworld	37 to 42	100%	All places
Pidgeotto	Flying pokemon in the sky	3 to 56	100%	All places
Pidgeot	Flying pokemon in the sky	3 to 56	100%	All places
Rattata	Walking in the overworld	37 to 42	100%	All places
Raticate	Walking in the overworld	37 to 42	100%	All places
Oddish	Walking in the overworld	37 to 42	100%	All places
Gloom	Walking in the overworld	37 to 42	100%	All places
Vileplume	Walking in the overworld	37 to 42	100%	All places
Tentacool	Surfing in the overworld	37 to 42	100%	All places
Tentacruel	Surfing in the overworld	37 to 42	100%	All places
Chansey	Rare spawn in grass with lure or catch combo	37 to 42	100%	All places
Tangela	Walking in the overworld	37 to 42	100%	All places
Staryu	Surfing in the overworld	37 to 42	100%	All places
Starmie	Surfing in the overworld	37 to 42	100%	All places
Magikarp	Surfing in the overworld	37 to 42	100%	All places
Articuno	Rare spawn in the sky with lure or catch combo	3 to 56	100%	All places
Zapdos	Rare spawn in the sky with lure or catch combo	3 to 56	100%	All places
Moltres	Rare spawn in the sky with lure or catch combo	3 to 56	100%	All places
Dragonite	Rare spawn in the sky with lure or catch combo	3 to 56	100%	All places