Hm... There are actually quite a few cries in the Diamond, Pearl, and Platinum games that have really been amazing.
1.) Arceus. It's a bit of a crazy cry, but awesome nonetheless. It sounds like a mighty roar mixed with the echoes of the universe...
2.) Dialga. Definitely a great roar for the god of time.
3.) Giratina. A ghostly "Bishaaaaan!!" that's simply great.
4.) Shaymin Land and Sky Formes. Both are great, and for those that haven't heard Sky Forme's cry, I'd say check it out.
5.) Ho-Oh. Definitely fitting for the rainbow bird.
6.) Glaceon. It's almost chilling, and fits perfectly with the Pokémon.
7.) Probopass, another fitting cry.
8.) Vaporeon. Another great one.
9.) Nidoking. One of my very favorites, it's been awesome since Blue.
10.) Kyogre. Perfect cry for the king of the waters.