I must say, I'm very surprised by some of these picks.
Mankey can easily own Brock's gym, even without a lot of training (and that's if you're too lazy to get Metal Claw on Charmander).
I caught a lv 14 Gloom while hunting for Abra, and without any training it easily beat every single thing in Misty's gym.
If you got Geodude way back in Mt Moon, he can solo Lt Surge. He's immune to electricity so even if he's under leveled he handles them.
And for Koga, does no one take the time to catch an Abra? He'll evolve long before reaching Koga, and he can 1-hit poison types twice his level with Psychic attacks, on top of which he's one of the fastest pokemon in the game.
But SABRINA! There's really nothing in the game that works against psychic types, and her pokemon are insanely high leveled. I only managed to beat her because Gyarados is a beast and bite is a dark move now so it's boss, and I was just able to evolve Charmeleon into Charizard with a Rare Candy, and flamethrower eats everything. I still had to revive Charizard THREE TIMES! And she beat me three times before I was able to figure out what to do.
In Emerald, I didn't have any real trouble until the normal gym with the main character's Dad. Those were some seriously over leveled normal types.