GSC because you can go back to Kanto and everyone knows that Kanto has the best region and Pokemon and eeeeeeeevvvvvvvverrything. I mean, like, you can get 16 badges! That's twice the amount of badges as all the other games! That's so much more to do! And then Red, like, omg, he had 80-something Pokemon! Nothing was more enjoyable than grinding mid-level 40 Pokemon over and over!
*cough*
GSC would probably get my vote for most revolutionary/game-changing of the franchise, but it's a buggy and unbalanced mess. They only somewhat fixed it in the remakes but they still managed to fuck it up. By no means a bad game, probably my third favorite generation in the series.
At this point, probably Platinum.
Region - the mountain essentially splitting the place in two, the northern snowy summit, the trio lakes, Stark Mountain/Battle Frontier island. It was varied and put together nicely. Hoenn is still my favorite, but Sinnoh is close and reminiscent of it. Kanto and Johto are both extremely plain to me, being generic grassland-like areas - the newly added routes to the Safari Zone in HGSS probably being the most interesting. Unova is really nice, especially since the seasonal changes keep them fresh.
Storyline - I really liked the Distortion World's inclusion, though I didn't care too much for Team Galactic. I remember their headquarters being really cool though. I liked Magma/Aqua the best probably, because you actually see their plans come into fruition more so than anyone else, and eco-terrorism is different.
Culture - Sinnoh had a lot of folklore that gave itself more of an identity. I would expect this the most from Johto, but Sinnoh had much more.
Music - the music was very different in DPPt. I wouldn't say it's the best in the series, but it was nice change. Was much more atmospheric than previous generations, incorporating jazzy and western-feeling tunes and just having a much broader range of styles in general.
Poketch - best app in the game, bar none. The Pokegear and Pokenav were terrible in comparison, mainly because the phone mechanic was terribly implemented. Vs. Seeker took care of what the phone did ten times better and didn't rely on me waiting at its beck and call for me to have rematches.
Also, the Pokeradar. This was such an amazing item that makes me continuously come back to playing Platinum.
Worse thing about DPPt was probably the Pokemon themselves. Not many new ones in retrospect as they went back to make older Pokemon like Magmar and Tangela even uglier than they already were, but certainly enough were I can make a regional team comfortably. Black and White made up for it though.
Edit: oh yeah, forgot to mention the Underground. It was fun just going down there to dig up fossils and shards, even if I didn't need them.