Best Overall Experience Training Locations
If you're looking to get your Pokemon to gain the most experience possible, look no further than Elite Four. Unlike the rest of the Trainers found throughout the game, only the Elite Four can be challenged as much as you'd like. And since their Pokemon are some of the highest leveled in the game, all in the high fifties, you're Pokemon will gain a boatload of experience.
If you're looking to gain the most experience from Wild Pokemon, then your best bet is Victory Road (Pokemon Levels up to the mid-fourties) or Cerulean Cave (Pokemon Levels up to the fifties). They have the highest leveled Wild Pokemon in the game; however, they still won't add up to the what your Pokemon can gain from the Elite Four.
It would've been much easier if you could have the items the Lucky Egg and the Exp. Share, but, unfortunately, these items weren't created until the Second Generation. The Lucky Egg would've allowed you to gain more experience in battle if your Pokemon was holding it, and the Exp. Share would've allow Pokemon that don't go in battle to still gain experience. But since they don't exist back in the first generation, you have to train your Pokemon the old fashioned way.