For me it's probably the axis. I really like the compression lock on the pm2 as well. I am a lefty so that eliminates anything that's not ambi or lefty friendly for me. The two mentioned above are extremely easy to open, smooth, and you don't have to place your fingers in the path of the blade to close. Lockup seems to be as solid as other mechanisms. Just My opinion but the liner locks, and back locks just don't operate as smoothly. I've warn out the assists on most of my assisted knives. Just seems the axis nailed it.