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While I dislike Diamond and Pearl, Platinum does a good job fixing the problems with those(notably the lack of variety in wild Pokemon). You still need a buttfuckton of HM users, though.
Black and White are very story-centric, but if you don't mind mashing through walls of text and going to useless locations, they're great games. If the story wasn't dragging me around the whole game, I'd say it's entirely better than Platinum.
I haven't played B2&W2, so I have no idea if those are any good.
Although you said you can't afford X and Y, I've gotta say that I enjoyed them the most out of every game in the series.