I would say it's a hard choice as there are so many, but ever since I discovered the Star of Azura, I'd say the decision is now pretty easy. Well, not the original star, but the tainted Black Star. The Star of Azura acts as a soul gem trapping the souls of creatures you cast soul trap on and kill. However, the defiled Black Star is much more practical, and deliciously evil! It's especially useful to me as I follow the dark arts. Unlike regular soul gems, black soul gems can trap human souls, which are encountered more often, therefore you usually never run out of souls to enchant or recharge with.