Your own little world to become the master of
Gen 1 has its flaws, but I think it really does handle "boss" fights the best.
Pokemon like Onix, Alakazam, and Dragonite are really tailored to be used against the player, and serve as tests along the way.
The rate at which the player gets access to new Pokemon feels really good.
And, damn, if the music and spritework isn't just charming.