For me, it's got to be Gold, Silver, and Crystal. They have included many new features, and had also given us access to Kanto throughout the end to top it all off, which I believed to be pretty cool. As for first generation games, I'll likely have to side with Yellow; although you cannot get any original Red, Blue, and Green starter Pokémon from get-go, you're still able to get your hands on them as you progress through events and whatnot. Also, Professor Oak gives you Pikachu, which follows you around wherever you go; simply adorable!