Easily the quietest air-pistol I have owned is the Weihrauch HW70. The power is not high, but as it is a springer it makes it easier to shoot. Excellent trigger and sights; the grip is a bit slippery but that's not difficult to fix.

If you have the money and you can find one, the Walther LP53 is also a quiet pistol. Anything with a big blast of gas or air is going to make a bang, so really SSPs, CO2 and PCPs aren't going to cut it unless you fit a 'orrible sound moderator.