Rate my team on Pokemon Diamond please.?

You have just started right, then how do you have all of those good Pokemon? Anyway, that's not important. Your team looks really good and the levels are evenly spaced and you have good Pokemon. My advice is to add a Fire type in your team for extra "oomph" if you know what I mean. I would suggest Camerupt or if you like legendaries, then I suggest Heatran. As for the final move on your Drapion, may I suggest Spikes or Toxic Spikes. Perfect for rivals that switch Pokemon. Dialga's moveset is really awesome and the best move at the moment would be Flash Cannon. There is a better move, but it learns it on level 90 so you will have to do some training. The move is called Aura Sphere by the way. Other than that your team is pretty much perfect and I would rate it, believe it or not, 9.5/10. The reason I am not giving full marks is because you need to train Gastrodoon and Drapion a bit more, but other than that, excellent.