HKS Hi-Power. I have it on my red MKIV. Many years ago, I had the GReddy SP on my black MKIV. The HKS Hi-Power (75 mm) is at least as quiet as the GReddy SP, with possibly a slightly deeper tone. My red car is BPU with all the plusses (I think).
The ATR exhaust is pretty quiet as well, way too quiet for me. They only time you could really tell it was louder was in WOT, but even then it wasn't as loud as my car with a Carbon Ti exhaust at idle.