Pokemon Platinum: The "Complete Package".
Aside from these online features, there is also another newly added feature called the Wi-Fi plaza, which allows you to meet up with people online, without exchanging those bothersome friend codes. Of course, since there are no friend codes required for this activity, there is unfortunately no battling or trading, so this mode gets a little boring after a while.
However, the reason why I bought Platinum was mainly because I believe that Platinum is the only Pokemon game that gives you the "complete package." In Platinum, EVERY SINGLE POKEMON is available for you to catch, train, or trade with. The reason for this is because Platinum features a variety of Pokemon from ALL four of the Pokemon generations, and features them in the wild for you to capture. This means that you are able to construct the "Pokemon team of your dreams" very easily, since a lot of the Pokemon are available without having to trade. Of course, Platinum does not have all the Pokemon available in the wild, so there is reason to trade with other people, and that adds to the total experience in the long run.
In conclusion, Platinum is truthfully the game that everyone interested in Pokemon needs to get. It features all the features of Diamond and Pearl, plus more. If you could only buy 1 Pokemon game, then I would highly suggest buying Platinum above the rest. It is a game that will please old and new gamers alike.