I always got one of the two main games then the counterpart, and tended to find Crystal and Emerald more enjoyable than their main counterparts. Graphically they do a bit more, Platinum has some fantastic move tutors and from what I've seen some areas (Eterna Forest springs to mind) is fantastic.

I have neither Diamond or Pearl yet, so if I were to get a game logically I'd grab Platinum. But after seeing the unavailable Pokemon....I'm not convinced! I really like Murkrow, Misdreavus and Stunky, and even Glameow is ok. Having acceses to none of them is a bit lame :/