Well, it's the Original Series!(Indigo League, Adventures in the Orange Islands & Johto seasons)
Those were just too EXCELLENT and I watched'em a lot when I was even younger...Ash's old Pokémon, one of the best rivals- Gary Oak, the original and my favorite PokéTeam(Brock & Misty along with Ash & Pikachu) and a younger Ash! Many reasons, including the ones I just told. And if I have to choose between the three adventures in Original Series, I would choose Johto, since that was just a unique season and I have a lot of Johto memories.