I'd say GSC. It introduced:
- Breeding. Seriosuly, you can't say this wasn't an important development, Egg moves are important for many Pokemon and the ability to breed for ideal IVs, Natures and Abilities is extremely useful. (Yes, those things weren't available at the time, but Breeding made them much more easy to control once they were introduced).
- Entry Hazards. Stealth Rock is big, but I honestly don't think we'd have Stealth Rock without Spikes.
- The Special Attack/Defense Split. This explains itself.
- Hold Items. Again, although it wasn't until later generations that Choice Items, Life Orb etc were intorduced, it was this that made the way for them. Plus leftovers is still a great item.
- Weather. Who doesn't love weather? Rain Dance Teams, Sunny Day Teams, Sandstorm Teams and Hail Teams all owe their existence to the introduction of weather. Again, it's not until later that these really start to shine, but GSC made the way for bigger things later.
- Baton Pass. Oh the joy...
- Steel and Dark types, and of course the modifications to how typing works. These balanced everything more and made things better. Steel types are a massive defensive force even today.
Of course, the others did pretty well too. RSE introduced Abiloities and Natures, but as I mentioned, GSC's breeding made these way more accessible.
RSE also re-worked IVs and EVs - something that I can't credit GSC for helping.
DPPt greatest asset was the Physical/Special Split, perhaps even more important than GSC's Special split. Honestly I don't see why it wasn't this way in RBY - in fact, when I was younger, I thought it was. (Bite was Special in GSC and RSE, WTF?). Stealth Rock is pretty big, but it's really only a descendant of GSC's Spikes.
Anyway, I think that's it from me...