Well, evolutions are tied to the story, in a sense. Hopefully, nobody has a Swampert when facing Julia, they would most likely have a Marshtop, which is why I was suggesting talking about when certain evolutions are viable.
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Slakoth -> Vigoroth -> Slaking
Tier: A or B
Availability: Early (headbutt required)
Stats: Highest of available Pokemon
Movepool: Excellent
Slakoth is found around the level that it evolves into Vigoroth, so most players will not have it long. Because of its attack stat and normal STAB, Slakoth's Scratch (or Facade if you have the TM) should KO most Pokemon around it's level in 2-3 hits. Of course, due to Truant, this will take 4 turns minimum.
Vigoroth is a blessing in disguise. Coupled with Facade (or Slash), Slack Off and Faint Attack, this Pokemon is capable of doing decent damage to almost every Pokemon in or around its level while being able to heal itself upon taking damage. Vital Spirit prevents it from being put to sleep and trolled, making it helpful against Pokemon with Sleep Powder, Spore, Hypnosis, etc.
Slaking is a Pokemon that can be a savage when used properly. While unable to sweep teams due to Truant, Slaking establishes its worth as a revenge killer and Cofagrigus' worst nightmare (acquiring Mummy if Slaking hits it or it hits Slaking.) In Double Battles, the player's Cofagrigus can give Slaking its ability, making Slaking a force to be reckoned with on the battlefield. Because of its vast movepool, Slaking's type coverage is excellent, making it dangerous when switching into battle. On that note, due to Slaking's bulk, it should have no problems switching in unless it is underleveled or hit by a Focus Blast.
Overall, Slaking lacks the potential to sweep, which hinders its usefulness slightly. However, because the player can switch freely in single battles after knocking out a Pokemon (which should happen in 1-2 hits due to Slaking's monstrous attack stat), this problem goes mostly unnoticed until double/triple battles occur. For these reasons, I believe Slaking deserves an A or B rating, but does not merit a C or lower.