The word "most ascertainable" refers to something that is easily determined or able to be discovered. Its antonyms would be words that indicate difficulty, uncertainty or impossibility in determining something. These include words like vague, ambiguous, uncertain, elusive, indeterminate, incomprehensible, mysterious and intangible. These words are used to describe things that are difficult to grasp, understand or pin down. It may be challenging to determine a clear meaning or solution for problems that are described using these antonyms of "most ascertainable." Conversely, using synonyms such as clear, evident or obvious can make problems easier to solve, decisions easier to make and communication more efficient.