Panasonic, hands down the quietest. Mine are quite old , but by far a lot quieter than the newer unit. I can't speak for cost, but their bath fans a virtually silent and are a little more money than the box store offerings.

Panasonic used to be the quietest but they got out of the window AC business years ago. Sharp AFS60RX and Frigidaire FRA065AT7 have the best noise specs of current models (48db). How it is installed matters too. You will want to seal the window tight and lots of people drill a hole in the pan to allow the condensate to drain off and eliminate the swishing sounds.

Check the ratings on the major sites like sears, lowes, homedepot and amazon. Don't make a decision based on just db's. Anything within 10db's or so is not all that noticeable. You may want to place more priority on sizing and features. Also be careful if you decide to drill the pan. In many cases this is not necessary as the manufacturer has designed a means of draining already.

Here is a link that might be useful: Portable AC Reviews