Much depends what you are looking for.
M&K are very popular from the US, given the dollar rate - but tend to be raspy, metallic, and somewhat loud. I had one on my 3.2 Targa Sport for about two months, but after setting off car alarms and waking the neighbourhood, changed to a standard Dansk.
http://www.mkexhaust.com/The Dansk is far more subdued noise-wise, but has excellent gas flow and performance characteristics.
(you can also get a sport version)
http://www.autofarm.co.uk/parts/Dansk_exhaustsH&S have vast experience, and their muffler sound is much deeper. Straight muffler, 4 wks delivery, £350 + vat.
https://www.haywardandscott.com/results.asp?category=16Richard Thomas near Birmingham supplies most of the exhausts for the VW crowd. He has recently started making 911 mufflers so is worth a call. Made to order with stainless steel flanges (check other producers for that) they are beautifully made. Sound is deep, smooth, and refined, but not excessively noisy - in fact very liveable with and still produces the distinctive flat six wail when crossing into 4500 +++!
http://www.turbothomas.com/headers.html