The term filamentous refers to something that is thin, thread-like, or fibrous in shape. When describing something as "most filamentous," there are many synonyms that can be used to convey the same meaning. Some possible alternatives include slender, wispy, delicate, spindly, wiry, stringy, fibrous, hair-like, and fine. Each of these words paints a slightly different picture, but they all capture the same basic idea of something that is thin, elongated, and perhaps slightly fragile. Depending on the context of the sentence, one of these words may be a more effective choice than others.