In my experience, most flash hiders and compensators tend to redirect the sound back towards the shooter, this tends to make the sound (IMO) greater for the shooter. So, in essence, to reduce the perceived sound level (by the shooter) a plain/ target barrel is the best bet.
If any compensator or FH actually has any reduction in total DB, I am not 100% sure. But I think the difference is negligible.
After that, if you are NOT in mass, and want to jump through the hoops.. I would probably look at Gemtec.