Wow
last time I was here was at least a couple years ago and I insisted that it was impossible to create an "ultimate team" of any kind
and I am here, once again, to say that it is not only impossible to create an ultimate team, but "even more impossible" with the advent of Diamond/Pearl.
There are so many threats to consider, and so many more items, moves and abilities that create so many more options. I could create a team that came as close as possible to being unstoppable in the current metagame, and a week or two later I would already be having trouble continuing to consistently rack up victories. In a matter of like two days, Walrein went from "meh lapras" to a common threat. And, in a few more days, you can expect that it will be replaced by something else as it "goes out of style" or is simply replaced by something better.
It's as simple as that.