From my experience with an old Marlin 25N (22" bolt action), rated from quietest to loudest:
CCI .22 CB Long: actually sounds quieter than a pellet gun, you can hear the firing pin drop, but accuracy kinda sucks (2" at 25 yds)
CCI CB Short: actually slightly louder than the Long, with accuracy virtually non-existent
Remington 22 LR Subsonic: still very quiet (as loud as high-power piston airgun), but very inconsistent with many FTF and fliers
Aguilla 60gr SSS - the WHACK of the projectile hitting the bullet trap is louder than the report, good accuracy (1" at 50 yds with occasional flier)
CCI Standard Velocity - louder, but still subsonic (no crack), good accuracy (.75" at 50 yards)
Wolf Match Target - even louder, probably transsonic, excellent accuracy (.5" at 50 yards)
Anything 22 LR high-velocity (loud supersonic crack)
CCI Stinger (even louder CRACK, makes your ears ring)
With that in mind, my favorite low noise round is the CCI Standard Velocity, which combines good accuracy with low report.