Which silencer is the quietest?

As far as I am aware, there are several silencers, but I'm not sure which one is the quietest. I'm trying to do a mission silently, but the standard silencer on my pistol alerted the guards and I failed to do the mission stealthily.

Are there any quieter silencers than the standard one?

Every silencer is totally silent. "Threat" is essentially the new "noise" stat, and for all silencers, the "threat" drops to zero. The main differences between the different silencers come in their damage and concealability penalties.

There are a number of ways to do things more quietly though. The Shinobi ace causes guards to make 95% less noise when killed by melee or a silenced weapon. Also remember that for most guards, your melee strike will take them out in one (maybe two) smacks, and is even quieter. Careful not to get cuffed though!

At the end of the day, stealth comes down to practice. Sure the right equipment and the right build helps a lot, but at the end of the day, even with the best equipment and skills, it's still very easy to mess something up. And you have a very small margin of error with stealth, depending on the mission.