In case you don't know, quiet raise sp. attack and lowers speed. If it isn't a good nature, can you tell me the natures i'm okay with? Not just the best one.
Quiet is okay. I gives Suicune extra power while taking off a stat it doesn't desperately need. However, natures such as Calm or Bold would probably be better since it makes its already good defenses even better.
Best nature would have to be Bold, followed by Calm. But Quiet is a nice nature, especially if you give him good Defensive EV's and with the help of Calm Mind, you can Surf your way through practically any Pokémon.