Crystal was the best Pokémon game before X/Y. It set a new standard (that wasn't even seen in G/S) for story in the franchise. Additionally, this is where the first animated sprites were. The introduction to steel and dark types helped keep the meta balanced, and it did such a great job it didn't get another type balance until X/Y. In my opinion, Crystal remains the most replayable Pokémon installment to date.