The third and the fourth, naturally. Why? Well, many reasons.
First off, my first Pokemon game was Sapphire, not Blue or Red or Yellow. Sapphire. Then I bought Ruby. That in itself is very significant. I was exposed to other Pokemon a while before that, but it was really RSE the generation that was first for me. I wasn't a fan before that, and all the Pokemon I had seen before were in TCG cards, which I avidly collected until I decided to buy the games and a GBA.
And fourth because while the storyline isn't great, and many of the Pokemon are just evolutions, DP has been the best games ever in terms of mechanics. Do not dare tell me otherwise. A lot of Pokemon were unable to make full power of their stats, movepool and abilities because of the whole physical or special thing. DP changed all that, plus many other things as well, such as increasing the movepool of many Pokemon. Who of you can say Charizard was as good in GSC as he is in DP? None of you. Charizard is now fully capable of being physical, special or both, and he's got a much bigger movepool now, period.
Lastly, also the fourth because while the Pokemon are not original, they're useful evolutions in most cases. Before DP, it pained me to raise Togetic, Misdreavus and Roselia knowing full well they'd be able to do so much more if they evolved. DP brought that. Before DP, types like Grass/Ground and Poison/Dark were but dreams of mine. Today they're a reality. Before DP, there were no legendaries I really liked. But that's all in the past now as I love Shaymin, Mesprit, Cresselia and Giratina. :3
So yes, long story short, 3rd and 4th gen all the way, babeh. :D