Ideally, you'd want proper tires to be used with proper suspension so that the performance and life of the tires can be maximized.
But if you had to choose one...
R comp tires will make the greatest difference by far. Try Continental scrubs from the Grand-Am GS class. http://www.turnermotorsport.com/p-44...-27535-18.aspx
They are the most cost-effective r comp tires money can buy.