The best build quality system on the market for the FRS is without doubt the system made by Primary Designs although you would expect that based on the quality of work they do for F1, btcc, superbikes, etc, etc As a result though it will be the most pricey with a full system setting you back at least £800 with decat or over £1k if you want a sports cat

I've got a custom silenced PD on mine & its excellent although you wont pass many track days noise regs with it fitted (mine is over 105db )

Value for money vs build quality blueflame are very highly rated on the RSOC forum as one of the guys there had them develop a system off his car including manifold - otherwise your other options will be the miltek / mongoose & even scorpion variants that are out there