I went for a Milltek after my STD system rusted away. I dropped on with mine to be fair and brought if from euro car parts for a good price with a discount code.
Demon tweaks sell the full range of Milltek for a decent price too.
I went for the non resonated cat back with 4" tips. The build quality is very good, all fits perfectly with no knocks, bangs or leaks. Sound wise it's very quiet while on motorway, but a bit boomy when you give it some beans. Sounds good thou.
As for other makes, Jetex make a full system that's supposed to be louder and is cheaper than the Milltek but it starts at the sleeve joint on your STD downpipe, not the cat.