There is no such thing as best unless you state what you want to do with the car.

If its a road car then a set of konis with decent springs that are stiffer not lower is perfect for ride, braking and steering.

The best suspension for the S1 is RS16i billys with the cup for the std springs cut off and converted to coliovers by taking them apart and then welding on the sleeves and adjusters that burtons sell for about £50. This is better suited to a high power fast road or track car. This is what the race version on the S1 and RS16i would have done/used.

Next option is £2k+ which i doubt many people would want to fit. So i wont waste the time typing.