Also, my guesses for Slakoth's tiering would be Slakoth (Vigoroth) to D Rank and Slakoth (Slaking) to E Rank.
Availability: Route 104
Typing: Pure normal is highly oversaturated and never gets any positive matchups, especially with Roxanne and Brawly coming at the beginning.
Stats: Slakoth has horrid stats even for this point in the game, especially its bad speed, Vigoroth has middling attack but acceptable speed and Slaking's stats are nothing short of fantastic.
Movepool: The Slakoth line has a ridiculously wide movepool - Yawn is the earliest sleep move in the game, which is very handy for catching things (the wilds will fall asleep after you switch out Slakoth, which is rather nifty), Encore is helpful for certain battles (usually when the opponent tries to set up on you), Slack Off means you're less reliant on healing items and you have a massive attacking movepool, usually Fighting/Dark coverage to cover your physical Normal STAB and the elemental beams to hit certain weaknesses (even with Vigoroth's crappy Special Attack, an x4 effective move in the right place can hurt).
Major Battles: It doesn't do all that well vs. Roxanne and Brawly, but afterwards, you can SE the gyms by switching its TM loadout - Bulldoze for Wattson/Flannery, a variety of Fighting moves for Norman, BoltBeam for Winona, Feint Attack/Shadow Claw for Tate/Liza, various things for the teams, Thunderbolt for Wallace, the Fighting moves for Sidney and Glacia, Shadow Claw for Phoebe, Ice Beam for Sidney and Earthquake/Fire Blast for Steven.
Other: While Vigoroth is held back by middling stats (its bulk can be somewhat corrected with Eviolite), Slakoth itself and Slaking have an even bigger problem: Truant. This ability prevents them from acting every other turn, leaving them easily KOed in longer battles and greatly slowing down the speed at which they can beat enemies. It's also in the Slow exp. group, which makes it hard for Slakoth to reach the relatively useful middle stage and to keep up in levels throughout the game.