I recently bought a Cold Steel Mini Tuff Lite and it was my first experience with the Andrew Demko designed Tri-Ad Lock. My next most recent knife purchase was a Benchmade Mini Grip with the Axis lock. Playing around with these two blades and using them a good deal got me thinking about what my favorite locking mechanism is and why. I haven't had a chance to use a Spyderco Compression Lock, a Button Lock, or a Paul Axial Lock, so this is an incomplete list. Here is a good thread from the Spyderco Forums on lock strength.
In order of preference:
1. Axis Lock: This is a pretty easy winner. Designed by Bill McHenry and Jsaon Williams, it does so many things really well. Lots of people fawn over it, not just me. First, it is completely ambidextrous, working exactly the same for right and left handed folks. It is also very easy to engage and disengage with the moving stop bar acting as the blade detent when the blade is in the handle. Finally, it closes without having the fingers getting in the blade path. It seems pretty strong, probably not as strong as the Tri-Ad lock, but more than strong enough for EDC use.
I will note that older Axis Locks were terrible. The springs that keep tension on the moving stop bar were not as strong and this causes all sorts of problems. Since the springs were upgraded the lock works great. I will also note that the Axis Lock works the same way, in general terms, as SOG's Arc Lock and the Ball Bearing Lock on the Spyderco Manix 2. It is also a relative of the Bolt Lock seen on the Spyderco Sage 3. All of them use some sort of movable stop pin to engage the tang of the knife and lock it between the moveable stop and the non-moving stop in the handle.
2. Tri-Ad Lock: the amount of abuse these locks can take is really incredible, but in all practical senses that is not necessary for EDC tasks. Lots of people really like non-locking blades such as the SAK Cadet or Pioneer, so I can't see how a lock this strong is necessary. It is certainly nice though.
The lock's design is really clever and the fact that it works just like a lockback is a plus (new features, no new actions required). Essentially it places a stop pin between the tang of the blade and the hook of a lockback. Shock to the blade towards the open direction (i.e. up) is transferred directly to the pin. Shock to the blade towards the closed position (i.e. down) is transferred to the lockback hook and then into the stop pin. Overall and excellent design. Also, unlike a lockback the Tri-Ad lock, at least the one on my Mini Tuff Light, really locks in with no blade play either way.
3. Liner Lock: I know this is a controversial choice, but it comes down to this--the average liner lock is better than the average frame lock. If all frame locks were as well made as the one on the Sebenza, then there would be no question which is better, but the fact is that even high end frame locks, like the one on the AG Russell Acies, exhibit all sorts of issues (such as wiggle during lock up). Liner locks are easier to make and easier to execute and therefore are, on average, better.
The strength of a lock, again in the EDC role only, is not that big a deal, so while frame locks might be better in hard use knives or tactical knives, in the EDC role the average liner lock is just fine. Even in hard use applications though, the Spyderco Military and the ZT350 seem to do quite well with a liner lock. I also like the fact that generally liner locks allow for smoother and faster deployment than the lockback design. They are not ambidextrous though and that can be an issue for some. Finally there is no worry about over extending the lock in a liner lock, like there is in a frame lock.
4. Frame Lock: With everyone hopping on the Ti framelock bandwagon there are a ton of frame locks out there right now. They just don't, on average, have as nice a feel as they should. The Sebenza set a pretty high standard and most just don't cut it, and a frame lock, being the child of perfect fit and finish doesn't function well (or as well) when corners are cut. A liner lock is a bit of a fool proof design, while this design is something like a racecar--when everything is perfect it is the very best, but when things are even a LITTLE out of skew...watch out.
5. Lockback: The lockback is simple. It is easy to use for both lefties and righties. But it is just not as nice as the other locks out there. There seems to be an inevitable amount of blade play in all lockback designs, meaning that only the highest end knives using this lock have good solid lock up. It also puts pressure on the blade through the entire opening, meaning that deployment is slower than on other locks. Finally, some locks, like the one on my Spyderco Lava have such strong springs that disengagement is tougher than it should be (I think this is a way of trying to force out all of the blade play).
In an EDC knife, just about any lock will do, but if I were building the perfect knife, it would either have an Axis lock or a Reeve-built framelock. I'll update this ranking as I get more experience with other locks. I am looking for a Compression Lock and an Axial Lock knife right now (especially the Axial Lock--how many locks are the pivot, the lock, and the opening mechanism all in one? Genius.)