If I were to buy ONE Pokemon game per (pkmn) generation, which would you suggest?

[HyrulianScrolls]

Gen 1: Red/Blue
Gen 2: Gold/Silver
Gen 3: Emerald
Gen 4: Platinum
Gen 5: Black/White 2 or HeartGold/SoulSilver (both have some of the best replay value in the entire series, can't decide)
Gen 6: X/Y