Adjectives

(s)foolproof, unfailingnot liable to failure“a foolproof identification system”, “the unfailing sign of an amateur”, “an unfailing test”
(s)unfailingalways able to supply more“an unfailing source of good stories”, “a subject of unfailing interest”
(s)unfailing, unflaggingtireless“unfailing loyalty”, “unfailing good spirits”, “unflagging courtesy”