"Most extendable" refers to something that is capable of being extended to a greater degree than other things. Synonyms for this term include highly flexible, extremely adaptable, remarkably pliable, very stretchable, and super-expandable. These terms all suggest a degree of elasticity that enables something to lengthen or expand beyond its usual boundaries. Other synonyms for "most extendable" include exceptionally versatile, exceedingly malleable, and particularly ductile. All of these words imply that something can be shaped or molded to fit a wide variety of needs or circumstances. In short, if something is described as "most extendable," it's likely to be highly versatile and responsive to change.