The antonyms for the phrase "most long serving" are numerous, and depend on the context in which the phrase is used. In terms of employment, antonyms could include "newcomer," "recently hired," or "rookie." In sports, antonyms may include "novice," "amateur," or "rookie." Additionally, in political contexts, antonyms could include "outsider," "freshman," or "newcomer." Ultimately, the opposite of "most long serving" is any phrase which connotes newness or lack of experience, and depends on the specific situation in which the phrase is used.