The words that serve as antonyms for the phrase "most well marked" are subjective and may vary depending on the context in which they are used. However, some possible options include "poorly defined," "vague," "indistinct," "faint," "inconspicuous," and "unremarkable." These words denote an absence or lack of clear and distinct features or characteristics that make something stand out or easily recognizable. If one wishes to convey the opposite of "most well marked," any of these words may be appropriate, depending on the situation and the intended meaning.