It really depends on which role.
If you are going with a full-on special sweeper set and want to hit hard with your attacks, opt for Modest (+SpA, -Atk), as this will boost your SpA without lowering any other important stats, since you wouldn't need Attack anyways.
If you are going special attacks all the way but want to be faster than your opponent without the use of a Scarf or any other speed-boosting items, you're looking for a Timid (+Spe, -Atk) nature. This will increase your speed and lower your attack, which as I said is unnecessary if you don't have any physical attacks in your moveset.
If you want to include physical moves as well such as U-Turn, then the options above wouldn't be a good decision. You would have to sacrifice another stat, preferably defense or special defense. If you think defense is less important, choose Mild (+SpA, -Def). Or, if you're willing to let special defense go, use Rash (+SpA, -SpD).