Treecko for Low/Mid. Yes, sure it's a starter, but it's horrible.
Availability: Starter. That's probably the one thing it has going for.
Stats/Movepool: Treecko overall gets a fairly solid physical movepool with Brick Break, Aerial Ace, Earthquake and Slash, except it's meant to be a special attacker, where it only has a dinky 70 BP STAB and maybe Dragon Claw if you bother detouring for it. Furthermore, none of its moves get past 40 BP until it learns Leaf Blade at 29 (around the time you collect the 5th Badge).
Major Battles:
Roxanne - Looks like a win for Treecko on paper, but unless you evolve it, get some potions and the like, Nosepass will overpower you and your crummy stats. At least Absorb works for the Geodudes.
Brawly - Lacks the power and moves to hit the fighters for major damage, whereas they can make short work of Grovyle for its poor bulk.
Wattson - Can possibly trade with Voltorb if it's lucky. It has Rock Smash now to hit the magnets SE, but that's too weak of a move and Magneton hits so hard that even the resistance won't save you.
Flannery - Do I really need to explain why this matchup is bad?
Norman - Finally, Grovyle has Leaf Blade! Unfortunately, Slaking can take Grovyle's STAB fairly well due to its massive HP. The same does not apply the other way around.
Winona - If you gave Treecko Rock Tomb way back, you might be able to KO Swellow. Otherwise, don't bother.
Team Aqua/Magma - Overall, all those Zubats and Golbats are a big impediment, and Numel/Camerupt can also hit it hard. Carvanha/Sharpedo can be beaten, and Poochyena/Mightyena really aren't a threat unless you're Ralts or so.
Tate&Liza - Sceptile can hit those rocks pretty hard - just be careful of Xatu and Solrock's Flamethrower.
Wallace/Juan - The one time where Sceptile can really capitalize on its type advantage. Just avoid fighting Tentacruel, Gyarados and Kingdra, who aren't bothered by Leaf Blade. Also, Milotic can take your hits and threaten with Ice Beam.
Wally - I guess you can defeat Delcatty and maybe Roselia if you have Aerial Ace. Magneton is a bit sketchy, Altaria walls you and Gardevoir has too much special bulk for Leaf Blade.
Sydney - You can take out Mightyena (which was already established as a wimp) and Crawdaunt. Absol is also doable.
Phoebe - Can trade with the Banettes and the Sableye - Dusclops has a bit much bulk. Some of these ghosts have Ice Beam.
Glacia - Next.
Drake - If you bothered to pick up Dragon Claw, you can actually pull through. Otherwise, you'll have to sit this one out.
Steven/Wallace - Against Steven, you can take on Claydol and struggle with the Rock-Types. Don't bother taking Skarmory or Metagross on.
Summary: Its early availability is offset by the fact it doesn't do anything useful until it learns Leaf Blade, at which point it's only okay at best. Better stick with Shroomish, Roselia, Vileplume or even Cacturne if you want a Grass-Type, which all have more accessible stats, better utility and require little to no switch training.
Availability: Starter. That's probably the one thing it has going for.
Stats/Movepool: Treecko overall gets a fairly solid physical movepool with Brick Break, Aerial Ace, Earthquake and Slash, except it's meant to be a special attacker, where it only has a dinky 70 BP STAB and maybe Dragon Claw if you bother detouring for it. Furthermore, none of its moves get past 40 BP until it learns Leaf Blade at 29 (around the time you collect the 5th Badge).
Major Battles:
Roxanne - Looks like a win for Treecko on paper, but unless you evolve it, get some potions and the like, Nosepass will overpower you and your crummy stats. At least Absorb works for the Geodudes.
Brawly - Lacks the power and moves to hit the fighters for major damage, whereas they can make short work of Grovyle for its poor bulk.
Wattson - Can possibly trade with Voltorb if it's lucky. It has Rock Smash now to hit the magnets SE, but that's too weak of a move and Magneton hits so hard that even the resistance won't save you.
Flannery - Do I really need to explain why this matchup is bad?
Norman - Finally, Grovyle has Leaf Blade! Unfortunately, Slaking can take Grovyle's STAB fairly well due to its massive HP. The same does not apply the other way around.
Winona - If you gave Treecko Rock Tomb way back, you might be able to KO Swellow. Otherwise, don't bother.
Team Aqua/Magma - Overall, all those Zubats and Golbats are a big impediment, and Numel/Camerupt can also hit it hard. Carvanha/Sharpedo can be beaten, and Poochyena/Mightyena really aren't a threat unless you're Ralts or so.
Tate&Liza - Sceptile can hit those rocks pretty hard - just be careful of Xatu and Solrock's Flamethrower.
Wallace/Juan - The one time where Sceptile can really capitalize on its type advantage. Just avoid fighting Tentacruel, Gyarados and Kingdra, who aren't bothered by Leaf Blade. Also, Milotic can take your hits and threaten with Ice Beam.
Wally - I guess you can defeat Delcatty and maybe Roselia if you have Aerial Ace. Magneton is a bit sketchy, Altaria walls you and Gardevoir has too much special bulk for Leaf Blade.
Sydney - You can take out Mightyena (which was already established as a wimp) and Crawdaunt. Absol is also doable.
Phoebe - Can trade with the Banettes and the Sableye - Dusclops has a bit much bulk. Some of these ghosts have Ice Beam.
Glacia - Next.
Drake - If you bothered to pick up Dragon Claw, you can actually pull through. Otherwise, you'll have to sit this one out.
Steven/Wallace - Against Steven, you can take on Claydol and struggle with the Rock-Types. Don't bother taking Skarmory or Metagross on.
Summary: Its early availability is offset by the fact it doesn't do anything useful until it learns Leaf Blade, at which point it's only okay at best. Better stick with Shroomish, Roselia, Vileplume or even Cacturne if you want a Grass-Type, which all have more accessible stats, better utility and require little to no switch training.