Choose the correct word for the blank in this sentence: John's great-grandfather was the ____ man I had ever met, and he was still healthy and active.

The term oldest is correct because it completes the thought in a logical way. Older uses the wrong ending, er. Most old has the correct meaning, but is an awkward fit and is not commonly used. The term most oldest is improper because the est ending indicates a superlative and there can only be one oldest person.