Based on what I've heard from the old 1200's, the Yoshimura Zyclone series and Holeshot Street series are the quietest aftermarket exhausts I've heard. Both are pretty subtle when idling or cruising along, but still let of a bark if you rap them out. Which one you choose would be based on which one you like the looks and/or price of.
I've been real pleased with my Holeshot 17" Street can. It's been on the bike for six years now. It seems my neighbors are OK with it, too. They're all retired and prone to complain about lots of piddly little things, but they've never complained about my bike. And since I've had the bike, I've worked days, swings and graveyard shifts, so the bike has been in/out at all kinds of crazy hours.