For me the most helpful would be breath control.Stutterers are often shallow breathers and just straight up forget to breathe when speaking.In non stutterers,breathing while talking is a spontaneous automatic thing and for some reason it isn’t for stutterers.Nothing has helped me more than thinking about taking a breath before each sentence,i take a breath through my mouth and make sure to breathe through my diaphragm which makes a night and day difference with my fluency as compared to when im not thinking about breathing.