The second best nutcrackers I've used, and I've used many nutcrackers, are the adjustable ones but there is one better: I use a modified 3½" bench vise. To modify it, first remove the movable jaw. Then, with a drill with a three-eights drill-bit, drill both jaw pieces by only the length of the drill-bit's tapered point with the holes drilled so they mate when the vise is re-assembled.
This unit is better than the adjustable nutcrackers because, if a given nut doesn't crack, just nudge the vise a hair tighter.
I put each nut in so that its connexus (in the case of dicotyledon nuts) is perpendicular to the vise jaws and start cranking. Butternuts, Juglans cinera, is the most difficult nut to crack, according to my experience, but, with the modified vise, I get 95% perfect whole halves.