The story is basically the same in every game from Emerald onwards, so you can cross that off your list as a factor really. You're going to get exactly the same story experience from everything on that list not Fire Red or HeartGold. If you want a story-driven experience then you definitely should not be buying a core Pokemon title, haha.
HeartGold is the best in terms of content - you get an entire region to explore for the postgame - but Platinum is very good, and I think it has the largest number of Legendaries you can catch in-game? Off the top of my head. Platinum also has a very nice Battle Frontier. If you're going to insist on story (although I can't imagine why) then you can't really get Black/White 2 because they're sequels, but the Pokemon World Tournament is lots of fun and they're not quite as dismal as their predecessors.
If you buy Pokemon Dream Radar as well you can have Legendaries in your party from the get-go too, and if you can even just borrow copies of the DS titles you can also get their box mascots (Dialga, Palkia, Giratina, Lugia, and Ho-Oh) in your game too. But if you can get your hands on an event Arceus you can have a level 1 Dialga, Palkia, or Giratina in your party from after the first Gym...and they'll follow you around. It's very cool having a Giratina follow you around.