Buckling springs are louder than all other mechanical types, Cherry MX, Alps (incl. Matias) or Topre.
I don't have experience with Alps clicky switches, but the Matias Quiet Click switches are actually very quiet, around the same as a Cherry MX non-clicky board (linear or tactile) with orings.
MX come in 3 main types, clicky (blues, greens and whites), linear (red, black) and tactile (brown, clear).
There are three places the switches make noise: any inherent click mechanism the switch has, bottoming out and release. The Matias has a click, but the bottom out and release are dampened by a rubber piece on the slider. MX linear and tactile have no click, but relatively loud bottom out and release noise. Orings dampen the bottom out noise.
I don't like the click sound of MX clicky switches, but I like the tactile feedback of the tactile switches (especially Clears). Linears have no tactile feedback when the switch actuates, which can be nice for some types of games, but I find I don't like typing on them.
Different keycaps can also change the pitch of the bottom out and release noise, I find thick, heavy caps to sound better (lower pitch) than thin, light caps.
My favourite switches are Clears with 62g springs, trampoline and latex modded for quietness (
http://geekhack.org/index.php?topic=50632.0), but stock Clears or Browns with orings are "good enough"
