HMs are a staple of the Pokémon franchise's early generations. Hidden Machines are necessary to traverse a region and explore it entirely, making them a necessity that most fans resent having. Indeed, HMs are often weak in battle, so many players opt for catching a specific Pokémon to act as a receptacle for them.
This HM mule will serve no role in the team in active battle and will only exist to learn the undesirable HMs, for example, Cut, Flash, or Defog. Fortunately, several Pokémon make ideal mules, thanks to their ability to learn multiple HMs and their accessibility early on in the game.
Empoleon
Empoleon is arguably the best Water-type Starter in the games. Thanks to its unique type combination--Water and Steel--Empoleon has a staggering ten resistances, one immunity, and only three weaknesses. As such, it would be a waste to use it as an HM mule, even if it does fit the mold.
Indeed, Empoleon can learn seven of the eight HMs in Sinnoh, including the three most useless: Cut, Defog, and Rock Smash. However, true to its real-life inspiration, Empoleon can't learn Fly. No player in their right mind would ever use a Starter as a mule, but Empoleon would be the ideal option to do it.
Hawlucha
Like Empoleon, Hawlucha has a unique typing, Fighting/Flying. And like Empoleon, using it as a mule would be a waste. Hawlucha is a Kalos superstar, ranking as one of the best Pokémon in the game.
Even so, Hawlucha fits the bill for an ideal mule. It is accessible relatively early, after beating the second Gym. Furthermore, it can learn half of the Kalos HMs, plus Rock Smash in ORAS. If a player catches a Hawlucha, the last thing they would do is use it as a mule, but they still have the option, making the luchador bird an even better Pokémon.
Sandshrew
Sandshrew isn't one of Generation I's most iconic Pokémon by any means. In fact, if it wasn't for the regional form it got in Gen VII, some players might not even remember it. Sandshrew isn't the best Ground-type in Kanto, but it does have some promise as an HM mule.
In Generation III, Sandshrew can learn three HMs, including Cut. If it learned Flash, it would become the ultimate HM mule in Kanto. Alas, Sandshrew settles for three, and its status as a mule does rescue it from obscurity.
Slowpoke
Slowpoke is an ideal mule in both Kanto and Johto. In the former, it is accessible as soon as the player gets the Super Rod, about halfway through the game. In the latter, there's a well full of Slowpoke in Azelia Town, the location of Johto's second gym.
In both regions, Slowpoke can learn three HMs, including the useless Flash. Slowpoke is also not a useless Pokémon because its evolution, Slowbro, learns some convenient Psychic moves. And like any Pokémon fan knows, the Psychic-type is absurdly powerful in Generation I.
Krabby
In all honesty, Krabby is a pretty forgettable Pokémon. Ash caught and used one in the anime, and not even that helped increase its popularity. At this point in the franchise, there are too many Pokémon; Krabby can get lost in the group quite easily. To make things worse, Krabby is an awful Pokémon with terrible stats.
However, this crab Pokémon can find redemption as an HM mule. In FireRed & LeafGreen, Krabby learns four of the requisite seven HMs. Krabby is also common in Kanto, although it does require the Super Rod. However, it should be a valuable ally for any player looking for a reliable mule.
Linoone
Linoone is part of the so-called "regional rodents," very accessible Pokémon in the game's early routes. In Hoenn, Zigzagoon--Linnone's pre-evolution--is so common in the games' first route that it might be most players' first catch.
On top of that, Zigzagoon evolves at level 20, which means that many players could have a Linoone before their first Gym battle. Linoone is a great HM mule, capable of learning half of the HMs necessary in Hoenn. Although it doesn't learn Flash, its knowledge of Cut and Rock Smash makes it an ideal mule for the Gen III games.
Quagsire
For all intents and purposes, Quagsire might be one of the most underrated Pokémon in Johto. Its Water/Ground typing leaves with only one weakness, one immunity, and four resistances. Its pre-evolution, Wooper, is accessible very early in the game and evolves at level 20. In all honesty, Quagsire should be a more widely used Pokémon, and it's a shame it isn't.
Quagsire can also work as an HM mule in the Johto games. It learns four HMs in Generation II, including two of the most useless, Flash and Whirlpool. Flash gets replaced with Rock Smash in Gen IV, which Quagsire can also learn. In short, this salamander-inspired Pokémon is an HM hero, and people need to realize it.
Crawdaunt
And speaking of underrated Pokémon, Cradawunt would also qualify. It suffers in the popularity department, even if it's one of the dual-type Water Pokémon that players should absolutely try. It might have below-average stats, but it has an intriguing type combination. Its pre-evolution, Corphish, is also easily obtainable with the Good Rod, adding to its convenience.
Crawdaunt is an HM beast. In every Hoenn game, it learns a whopping six of the eight HMs in the region. Crawdaunt doesn't learn Flash, but that ends up not mattering. Any Pokémon that learns Cut, Strength, and Rock Smash is an excellent HM mule.
Lombre
No Pokémon is perfect, but Lombre comes enough, at last as an HM mule. Lombre's pre-evolution is accessible in Hoenn's second route and evolves at level 14. Lombre is a version exclusive, available only in Sapphire and AlphaSapphire. However, Ruby and OmegaRuby can trade for one, and they really should if they're on the hunt for an HM mule.
Lombre learns six out of eight HMs, including the pesky Flash. It doesn't learn Cut, but that move isn't technically necessary to advance the plot. Lombre thus becomes the best HM mule in Hoenn and one of the best in the franchise.
Bibarel
In the pantheon of HM mules, none comes close to Bibarel. This Sinnoh legend is the very definition of what an HM mule should be. Bibarel's pre-evolution, Bidoof, is the regional rodents of Gen IV, meaning it is accessible from the very first route in Sinnoh. Bibarel's stats aren't great, but they don't need to be. Indeed, all it needs to do is learn HMs, and that it does.
Bibarel learns every HM available in the Gen IV games, except for Fly. Most players will use their team's Flying-type for Fly, leaving Bibarel for the other seven HMs. No other Pokémon in the franchise comes close to Bibarel when it comes to HMs. Indeed, this curious beaver is the undefeated king of HM mules.