If you ask 20 airgunners that question, you will likely get five different answers.
The most popular answer will probably be the Benjamin Marauder in .22 caliber. Followed closely by the FX Impact M3 in .22 caliber.
Side by side both rifles are very quiet, within a Db or so different on the meter, but they are two entirely different animals, aimed at two entirely different types of shooters.
The Marauder can be had in either an all weather synthetic stock, or a wooden stock, and has a fully shrouded barrel that uses the volume of the shroud to dissipate the air blast exiting the barrel, and has internal baffles to reduce the muzzle blast even further, the mechanical noise of the gun operaating is somewhat absorbed by the wood or synthetic stock.
The FX Impact M3, is almost entirely carbon fiber and metal, and acts like a tuning fork when fired, so there is little dampening of the mechanical noise. The M3 has a shrouded barrel with a DonnyFL moderator/suppressor on the end of the shroud, but the shroud is not actively used to quieten the rifle as the Marauder does.
The M3 also has a more tactical design to the airgun, where the marauder has a more traditional design.
The biggest difference is in the price, at the time of this writing, the Marauder sells for about $570, where the Impact M3 sells for $2200. The difference of the sound levels between the two rifles is negligible, but the differences of the price and shooting experience are worlds apart.