on mine, I didnt have alot of money to spend, so I installed 620 springs with 1" drop, new tie rods and center link, poly urethane sway bar bushings, gas charged shocks, and tightened the steering gearbox worm gear. It handles well for a car of its era, but eventually I would like to do a Shelby drop and quick ratio steering box.
Well to answer your question to the letter, the BEST upgrade available would be a coilover setup from TCP (I don't know how that works...do they have parts around anymore) or Global West. You could drop a couple grand eassssy if you want "the best".
Most of us however, go with the setup GEETEE mentioned. Some stiffer springs up front (usually 620s), stiffer springs out back (4-5 leaf), better shocks, bigger sways, subframes, and the good ole shelby drop.