Probably one of the hardest questions to answer tbf, like there's so many intros I love! When it really comes down to it, I'd say the original Indigo League has to be my favourite, purely from nostalgia and that objectively it's a really good intro that fits the series perfectly. Although any of Johto Journeys, Master Quest, Advanced, Advanced Battle and Battle Frontier could easily take second place, you can see I have a lot of favourites haha.