The term "most undefendable" refers to something that is easily attacked or cannot be defended against. Antonyms for this phrase may include "most defensible," which refers to something that is easily defended, protected, or justified. Other antonyms may include "most secure," "most safe," "most protected," or "most unassailable," all of which suggest a high level of protection or stability. Other related antonyms may include "most formidable," "most impregnable," or "most invulnerable," which suggests toughness or resilience against attack. Ultimately, the choice of antonym will depend on the context in which the word is used, and the degree to which defense or protection is called for.