My old ktec janspeed system was good. it was the 2.25" version rather than the normal 2.5" most aftermarket systems are. It didn't drone even with a decat and centre resonator removed. It was quite loud and raspy in that setup though, which i really liked. I think with a standard Cat and centre resonator it was one of the quieter systems.
Scorpion do a twin rear box system called the RS192, which with a centre resonator is suppose to be one of the quietest systems, but from experience the earlier single rear box scorpion did drone even with the centre resonator, so I'm not sure how good that would be.
From what's available these days, if buying new, I'd probably pay a little extra and get the Pure Motorsport system.