The original Red and Blue versions are the only Pokemon games I’ve never beaten. Maybe it’s because I was too young to really play them well, then by the time I got a handle on things, Gold/Silver appeared and I fell in love with that, never looking back.
Gold/Silver is far and away my most fond generation of Pokemon games. It could very well be my rose-tinted glasses as I was growing just old enough to appreciate it, but everything, with a two-region game and day-night shifts, seemed so big and grand. Ruby/Sapphire was very good, but it and many of the later one-region games just feel a little cramped in comparison.
I’d say my least favorite (and this is definitely a relative term, the game was still very good) would probably be Black/White. Simply because its story went into a higher “narrative bracket” by trying to cast doubt on the journey-while quickly and totally yanking back to the “it’s totally ok” message. But the gameplay was still very good.