I've got a Milltek Cat back on my 2014 GTI and I'm more than happy with it. Quality of workmanship, fitment (I actually managed to fit it with just the rear wheels on ramps it was so easy to fit), sound are all 100% spot on.
I had a Cobra Sport on my previous car (early Astra VXR) and whilst I would have another Cobra Sport exhaust, I would pick a Milltek over it every time. It was good, but not as good as the Milltek. Also, I don't know what grade of stainless they use, but it rusted way more then my milltek has.
I had a Scorpion exhaust about (ahem) 20 years ago on a Cavalier SRi, it was also good but couldn't comment on their systems now, especially as they've (I think) changed ownership since then too.
Mine is part number SSXVW230 and is the one with the front and centre silencer. Sounds great (not too loud and no drone while cruising on the motorway etc.).