I would definitely recommend Black/White (and Black 2/White 2 if you enjoy BW). It has a lot of the familiarity of the early Pokemon games, but also manages to pull off a sense of playing something brand new. BW does only have Gen 5 Pokemon until you beat the story, which is off putting to some people, but personally I think it enhances the experience. It kind of remedies the "once you've played one, you've played them all" problem you mentioned. BW also has a much more in depth story than all the Pokemon games preceding it.
BW2 are direct sequels to BW, so I'd definitely suggest playing BW first.
Platinum is nice, along with Diamond and Pearl, but they're a little sluggish, and I'd probably consider Plat one of the most difficult Pokemon games. It takes a bit of patience. Plat might not be the best game to pick up right away, but if you do get back into playing Pokemon games and want to give Gen 4 a shot I'd recommend it.