Pewter City Gym:

Leader: Brock
Pokemon: Rock (Onix and Geodude)
Best Pokemon to Use: Water (Squirtle)and Plant type Pokemon(Bulbasaur)
Worst Pokemon to Use: Electric or Fire Pokemon

Cerulean City Gym:

Gym Leader: Misty
Pokemon: Water Type (Staryu and Starmie)
Best Pokemon to Use: Plant type Pokemon (Ivysaur) and Electric (Pikachu)
Worst Pokemon to Use: Fire or Rock Pokemon

Vermilion City Gym:

Gym Leader: Lt. Surge
Pokemon: Electric (Voltorb, Riachu, and Pikachu)
Best Pokemon to Use: Rock Pokemon (Geodude or Onix) or Ground Pokemon (Diglett or Dugtrio)
Worst Pokemon to Use: Water, Bug, or Bird Pokemon

Celadon City Gym:

Gym Leader: Erika
Pokemon: Plant Pokemon (Victreebel, tangela, and Vileplum)
Best Pokemon to Use: Fire Pokemon (Charmeleon, Vulpix, or Growlith) or Any Psychic
Worst Pokemon to Use: Water & Rock Pokemon

Saffron City Gym:

Gym Leader: Sabrina
Type: Psycic Pokemon (Kadabra, Mr. Mime, Venomoth, and Alakazam)
Best Pokemon to Use: Ghost (Gastly, Haunter, or Gingar) or Bug (Butterfree or Beedrill)
Worst Pokemon to Use: Weak Defense Pokemon, Psychic, and Bug

Fuchsia City Gym:

Gym Leader: Koga
Pokemon: Poison Pokemon (Koffing, Muk, and Weezing)
Best Pokemon to Use: Any Pokemon With High Attack
Worst Pokemon to Use: Any Pokemon with little HP

Cinnabar Island Gym:

Gym Leader: Blaine
Pokemon: Fire (Glowlithe, Ponyta, Rapidash, and Arcanine)
Best Pokemon to Use: Any Water Pokemon with Water Gun, Surf, or Hydro Pump
Worst Pokemon to Use: Plant Pokemon

Viridian City Gym:

Gym Leader: Giovanni
Pokemon: Ground Pokemon (Rhyhorn, Dugtrio, Nidoqueen, Nidoking, and Rhydon)
Best Pokemon to Use: Water Pokemon or Plant Pokemon
Worst Pokemon to Use: Low Level Pokemon