Which oral antihyperglycemics are most effective in reducing hemoglobin a?

Pick your poison. Oral hypoglycemic drugs bring sugars under better control by different mechanisms of action. Fast lowering ones are typically drugs that stimulate increased Insulin production. With time though, there's only so much your beat up pancreas can do to keep up with the demands. Other orals work in different ways, sensitizing you to the effects of insulin. Not the most potent but friendlier2ur pancreas.
I think. You mean hemoglobin a-1-c. This is not a do it yourself project. Metformin is safe, effective and cheap for type ii variety. It and your diet blood sugar levels need to be watched by a doctor knowledgeable about the choices of drugs, and knowledge of your case.