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I just did a playthrough of all the 3rd games, so I'll put in my 2 cents.
Crystal, providing the Suicune subplot, definitely made it feel like more was going on than just Team Rocket and the Gyms. However, it added barely anything of note to the Kanto section of the game, which is understandable, given the technology.
Emerald is the game that added the most, by far. Move tutors, an entire new island featuring challenging opponents, interesting ways of obtaining previously un-obtainable pokemon, and a way to work in Rayquaza to the main story, while making the other legendaries available as well. What I really believe makes the game seem larger are the two teams. No longer do you play the ally of one team, battling against the other, but you have to fight both.
Platinum. Eh. Not bad by any means, but it didn't add anything. Sure, tutors and pokemon that should have been in the pokedex since D/P, but the rest is just a climate change. I will admit it has fantastic frontier brains, but they feel shoe-horned in, in comparison to Emeralds battle-facilities.
B2 and W2 I do not consider 3rd games, as they are continuations of a story, not a rehash like the others. I will say they should have had the villain as the final main story boss. Seriously, what they did with Ghetsis, Gamefreak should run with it for another 5 gens.
TL;DR 3>4>2