The phrase 'most fusing' can be replaced with several synonyms, depending on the context. For example, 'most confusing', 'most complicated', 'most intricate', 'most tangled', 'most puzzling', 'most perplexing', and 'most confounding' all convey the sense of something that is difficult to understand or figure out. Similarly, 'most entangling', 'most embroiling', 'most involving', and 'most enmeshing' can be used in situations where something is causing a person to become deeply involved or caught up in something, often with negative consequences. Other possible synonyms for 'most fusing' include 'most blending', 'most merging', and 'most integrating', which can be used in the context of combining different ideas, concepts, or elements into a coherent whole.