The Kanto region offers a variety of excellent hidden trainers to capture, train and use in your quest. This guide will let you know which are the best Kanto HM Slaves in FireRed & LeafGreen (And Gen I R/B/Y).
Nidoking

Nidoking is a Poison- and Ground-type Pokémon, also known as the Drill Pokémon. Nidoking introduced in the first generation, it is 1.4 m (4’07”) tall and weighs 62.0 kg (136.7 lbs). Its abilities are Poison Point and Rivalry. You can catch it at a rate of 45 (5.9% with PokéBall, full HP); its base friendship is 50 (normal) and the base experience yield is 227. It grows slowly. The egg cycles of these pokemon are 20 (4,884–5,140 steps).
Slowpoke

Slowpoke has a different Pokédex number and type, but retains all of its other abilities. It has low HP but high catch rates when using Poké Balls, and its base friendship is 50 (normal). It grows moderately fast. The egg group of this pokemon is Monster, Water 1. The gender of this pokemon is 50% male, 50% female. The egg cycles of the pokemon are 20 (4,884–5,140 steps).
Krabby

Krabby is a Water-type Pokémon and it’s best known for its rapid speed and aggression. Krabby was introduced in Generation 1 and it is nicknamed the River Crab Pokémon. Its height is 0.4 meters (1’04”) tall and its weight is 6.5 kilograms (14.3 lbs). Krabby has an Attack stat of 1 and a base friendship of 50. It evolves into Kingler at level 28. The egg cycles of these pokemon are 20 (4,884–5,140 steps).
Paras

Paras is a Bug/Grass type Pokémon, also known as the Mushroom Pokémon. It was introduced in Generation 1 and it evolved into Parasect starting at level 26. It stands 0.3 meters tall and weighs 5.4 kilograms. Its ability is Effect Spore and its Attack and Special Attack are 1 each. Its base friendship is 50, and its base experience yield is 57. It grows at a medium rate. The egg cycles of these pokemon are 20 (4,884–5,140 steps).
Sandshrew

Sandshrew is a Ground type Pokémon, and its Pokédex entry states that it lives on the desert sands of the Alola region. Sandshrew was introduced in Generation 1. It has a new Alolan form which was introduced in Pokémon Sun/Moon. Its Pokédex entry states that it lives on the desert sands of the Alola region. Sandshrew has the ability Sand Veil, which prevents it from being hit by sand-based attacks. It has a catch rate of 255 (33.3% with PokéBall, full HP) and base friendship of 50 (normal). Sandshrew grows at a medium fast rate and breeds in Egg Groups Field. The gender of sandshrew is 50% male, 50% female. The egg cycles of these pokemon are 20 (4,884–5,140 steps).
Farfetch’d

Farfetch’d is a Normal-Flying type Pokémon having keen eyes and an inner focus, making it difficult to catch. It was introduced in Generation 1. Its defiant ability makes it immune to status conditions for a short period of time after being caught. It has a catch rate of 45 (5.9% with PokéBall, full HP) and base Friendship of 50 (normal). It has a base Exp. of 132 and growth rate of medium fast.
Krabby

Krabby is a Water-type Pokémon and it’s best known for its rapid speed and aggression. Krabby was introduced in Generation 1 and it is nicknamed the River Crab Pokémon. Its height is 0.4 meters (1’04”) tall and its weight is 6.5 kilograms (14.3 lbs). Krabby has an Attack stat of 1 and a base friendship of 50. It evolves into Kingler at level 28. The egg cycles of these pokemon are 20 (4,884–5,140 steps).
Nidoking

Nidoking is a Poison- and Ground-type Pokémon, also known as the Drill Pokémon. Nidoking introduced in the first generation, it is 1.4 m (4’07”) tall and weighs 62.0 kg (136.7 lbs). Its abilities are Poison Point and Rivalry. You can catch it at a rate of 45 (5.9% with PokéBall, full HP); its base friendship is 50 (normal) and the base experience yield is 227. It grows slowly. The egg cycles of these pokemon are 20 (4,884–5,140 steps).