The best setup are the factory Bilsteins from a late model Grand Prix GXP. Had them in my old Regal GS and they were unbelievable. Unfortunately they are almost impossible to find and will cost you about $1300 brand new.
Other than that, people seem to like Vogtland springs (~ 1" Drop) and KYB Exel-G or AGX adjustable struts. If you want to stay stock height, look into the Impala 9C1 struts/springs, they will make the car ride a bit higher than stock, but are a bit firmer and ride nice. Theyre also pretty expensive unless you can get a good used set.
Whatever you do, dont buy Monroe Quick Struts, they ride softer than the factory stuff. I just bought a 2000 GS for my wife and the previous owner just had them put on and I absolutely cant stand how they ride, the car floats like a 70's Lesabre. My GS has 170k on the original struts/springs and I prefer that to the Monroes, absolute junk.
