To be perverse,

, and because I like a lock that was not mentioned I voted for "Something I missed (Let us know what it is!)"
The axial lock as found on the Paul Series II Model 2 knife by Gerber (e.g. final production run 1997 or the earlier Paul Model 2P (which I purchased in 1979). I refer to the dates this way as it gives an idea as to time period without worrying about ones made slightly earlier than when I purchased my first one or the ones put together from parts, by/for A.G. Russell, after the "final production run." This is United States Patent 3,942,249 issued on 9 March 1976.
This can be viewed here:
http://patft.uspto.gov/netacgi/nph-...&p=1&u=/netahtml/srchnum.htm&r=1&f=G&l=50&s1='3,942,249'.WKU.&OS=PN/3,942,249&RS=PN/3,942,249 .
This is such a long link I am not sure it will work, but if you go to the
http://www.uspto.gov/patft/index.html and search using the patent number it is easy to find, as opposed to searching using other methods. If you want to view the images, you may need to view the associated help file and get the suggested free download to get them to work.
To be fair and as a warning,

I note that there have been some complaints that pocket lint can defeat this lock. Perhaps so, but if it locks, it seems to stay locked. I have used knives with intregal locks, axis locks, lock backs, slip joints, liner locks, automatic knives, and none seem any faster or more convenient than this lock. It locks the knife open or closed and is ambidexterous. A simple squeeze allows one to open AND close the knife quickly and conveniently.
