Diligence best fits an octopus, due to its highly active and persistent nature in problem-solving, foraging, and manipulating its environment. 

Octopuses are known as industrious '8 Armed Einsteins' that consistently use tools, build intricate dens from rocks and shells, and tirelessly solve complex puzzles to get food. They are also described as persistent escape artists.  They frequently explore, manipulate, and attempt to leave their environments.

Female octopuses display extraordinary diligence by guarding their eggs continuously for months or even years, often neglecting their own survival. They defend the eggs from predators and constantly blast them with oxygenated water, a devoted continuous care, brooding her eggs longer than other species.  This commitment is so absolute that the mother typically stops feeding, resulting in significant weight loss and weakening, with many dying shortly after the eggs hatch.