Upper control arm bushings. As Chryco said, it is best to install front and rear bars for the most gains. The steering/suspension system is just that, a system. You need to determine what you would like to gain over the stock setup. As I said before, tires have a great affect on handling. The tire type and size, rimwidth and offset, shocks, springs,condition of the vehicles bushings and ball joints, weight of the vehicle, speed you are anticipating on the turns, in other words you need to get a baseline on what you have now, so you will know if the changes you are making are going in the right direction. It is just like choosing the components for an engine rebuild. You need to know what it did stock and where you want to go. Speed cost money, so you want to make good choices. If you are lucky and find someone using a similar car and finding out what they have learned in their buildup, you can save a lot of grief. A good place to get info is on the web. Key in what type of car you have and what kind of handling improvements you want and I am sure you can dig up a lot of info. Also try the different component Manufacturers sites to get recomendations and advice. Knowledge is power.
