It depends on who you're battling, but I'd say Butterfree, just because it has moves that can cause stat problems to your foes, such as SleepPowder, Stunspore, Poisenpowser, etc..
Chat with our AI personalities
Aerodactyl has higher attack then Crobat, while Cobat is superior in defense. Crobat also learns hypnosis, and is the fastest of any Pokemon to learn it. That being said, it depends on your team.
Put the Firered cartridge into the GBA slot of your DS (this cannot be done if you own a DSi, DSi XL, or a 3DS). Then put in your Soulsilver and turn in on. On the screen that you select your save file, choose mystery gift, and your wifi settings, there should be an option that says Migrate from Firered. The Pokemon cannot know and HM moves (like Fly or Surf). Choose six Pokemon to migrate and your all set to catch them. Fly to Fuchsia City in the Kanto reigon and head to where the Safari Zone was in Firered. There should know be a building where you can re-catch the Pokemon you migrated from Firered.
I'm not sure about leaf green but on firered it's in the basement of the poke mansion. I'm thinking it should be the same since firered and leaf green are so similar.
they should be at their highest stage and at least lv.40 to win easily
Go to Celadon City, and go into one of the big buildings. In one of them, there should be a women with a lot of Pokemon with her, talk to her, and she should give you the drink.