For the first generation games, the best Pokémon to use would have to be Chansey. It has great walling abilites, and with Softboiled, this Pokémon is nearly impossible to take down. Plus, it has access to Thunder Wave. Your opponent would be heavily slowed down when they're hit with this move, causing them to have a hard time trying to attack you. Pokémon like Rhydon and Golem could be a little threatening for Chansey, but they could be easily defeated if it has Ice Beam. (:
For generation two, Meganium is the best Pokémon, in my opinion. It's resistant to water, electric, and ground attacks, and it has access to various healing moves. With Leech Seed, Meganium will sip the health out of your opponent on every turn. With Synthesis, it's able to restore half of its HP without any sorts of trouble. If you utilize this Pokémon the correct way, you could stall your opponent for a lot of turns. n__n