For the fandom as a whole, I'm gonna say G/S. They took the formula that pulled in hundreds of thousands of kids and tweaked it to make it better. Pokémon was the coolest thing ever to begin with so no one thought anything could top it until... holy crap i can breed pokemon, there are baby pokemon that hatch from eggs, they can hold items, OH SNAP NEW DARK AND STEEL TYPES, new moves, new region, new cities, new starters, new evolutions, cell phone, and omg I can trade over my old Pokémon too. And so on.
After that innovation, though it might not seem like much anymore, everything else just seems so expected. Don't get me wrong, I like the other games (especially gen 4 which is my favourite thus far) but nothing can top the feeling of playing through G/S with your bros for the first time, discovering new (and yet "same old") things and sharing the experience.