After obtaining all eight Kanto Gym Badges and making it through Victory Road, you will arrive to Indigo Plateau where the Pokemon League is found. The Pokemon League is comprised of the Elite Four and Kanto Region Champion, five trainers you must defeat consecutively in order to become the next Champion. These trainers are the best of the best found within the Kanto Region, so be prepared for five difficult Pokemon Battles in a row.
After defeating the entire Elite Four, you will proceed to facing the Kanto Regino Champion, which just so happens to be your Rival. As you know from your many battles with him, your Rival has a very balanced Pokemon Team. Because of this, you will need multiple Pokemon of various types on your own Pokemon Team in order to defeat him. You will definitely need one Electric Type on your Pokemon Team, which will help defeat his Pidgeot and Gyarados/Blastoise (depending on his Starter Pokemon). You will also need a Water Type Pokemon, which will help with his Rhydon and Arcanine/Charizard. A Fire Type will also come in handy, especially against his Exeggutor/Venusaur. Theoretically speaking, a Bug Type Pokemon would do great against his Exeggutor/Venusaur and Alakazam, but since there are all that many powerful Bug Types in the Kanto Region, a Ghost Type is the best route to go for his Alakazam.
To recap, if you're looking to have an advantage over your Rival for the Kanto Region Championship, the best Pokemon Team to have will include at least an Electric Type, a Water Type, a Fire Type, and a Ghost Type. The final two spots on your Pokemon Team can go to two more strong Pokemon of different types than those already mentioned, furthering the balance to your Pokemon Team.
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