My best "flipper" is the CRKT Desert Cruiser. It's a big honking knife from the grip side, but it not only opens smoothly, it flips open with no problem. Some of the other CRKT flippers are fine is you flip them enough, but mine have frozen too many times to make them ultra reliable. I can flip my M21-14 a dozen times reliably, but if I pick it up cold I sometimes find that it doesn't flip authoritively into place. The Desert Cruiser I can open easily with one hand but, again, some of the others take a bit more effort.
I'm sure that if one were to take one knife and just practice with it that it will eventually loosen up; however, the Desert Cruiser didn't need any break in period. It's just smooth. But I've been carrying mine more lately because I've come to realize that rapid opening is crucial in self defense.