Does anyone know the best spot to train in Pokemon Diamond?

Hi, I have a Lv. 56 Torterra,a Lv. 55 Beautifly, a Lv. 47 Dialga, a Lv. 22 Staravia, a Lv. 14 Buizel, and a Lv. 5 Bidoof. All of the Pokemon except for Dialga have HMs, so basically I can't really take them away. I made it all the way to the Pokemon League but have not tried them yet, so I can fly to that spot. Can anyone suggest a place to train? I already know Victory Road.