The phrase "not least" is often used to indicate that something is particularly important or significant. Some synonyms for this phrase might include "especially," "notably," "significantly," "crucially," or "vitally." These words all convey the idea that something stands out as being more important than other factors, or that it deserves special attention or consideration. Other possible synonyms for "not least" might include "primarily," "chiefly," "above all," or "first and foremost." Depending on the context and the intended meaning of the phrase, different synonyms may be more or less appropriate.