What's a good replacement shock
That depends on you ride quality expectations. If you want a harsh ride that stiffly holds the car, then go for a performance shock. However, if you want a softer and smoother ride then go for an OEM shock that is similar to the originals. Most name brands offer many "strengths" to fit your car, some in AirRide configurations that are hooked into your car's compressor to manage the ride quality. Generally, for your GM car, look into AC Delco, Munroe, KYB, Koni and others by searching car part sites like RockAuto, AutoZone, Advanced, etc.