What is the best set to get for a stock 89 S5 Turbo II?

 

Had the Jacobs wires (from Rotary Performance) for years without issue. Though the boots aren't the greatest in that on the coil side, they really stretch to fit the coils. Next set of wires I do will be custom wire with a set of original Mazda boots.

 

I've used NGK plug wires on a number of cars with good results.
When I first got the FC and it was still stock, I was surprised how perfectly the NGKs fit (the sleeving is cut to use all the stock mount brackets and routing) and the boots all fit fine.

Later on, needing custom fit wires, I started with the NGKs and shortened to fit...again, with good results.
Concurrently I was also making a set of wires for the Z from a "build-your-own" Accel kit and considered doing the same for the FC but found it difficult to get the proper coil end fitting.
As Aaron noted, the FC has a large coil tower and most boots don't fit very well but the wire end fitting is also not a normal size (it's bigger).
The NGK boots/connectors are a perfect fit.

 

Probably overkill but I've had the Magnecor, I believe 8.5mm on the car for years.