My favorite is BC Racing because it can be customized and individual pieces are measured before assembly. I'm not one for slammed look, but I want stiffer suspension (with moderate lowering) that actually work well in most driving conditions. My rule of thumb is to target lowering of about 0.5-1 inch and specify spring rates about 25-50% stiffer depending on how I want the car to behave. BC Racing will automatically adjust the damper adjustment range.
Once the kit has arrived, I check the spring preload, about 1/16-1/8" is enough. I also check the measured numbers and use the units with higher numbers for the left hand side since the car has a higher corner weight on the left side. From there, install the coilovers and adjust height. Height adjustment is a different story since it could be done in several ways. The easiest is to set the left hand coilover units slightly longer (probably about 1/8" to start) than the right hand units. The correct (more time consuming and more expensive) way is to use corner scales and to simulate the driver's weight in the driver seat.
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