A 32-year-old woman presents with polyuria, polydipsia, and weight loss for 3 weeks. Blood glucose is 380 mg/dL, pH 7.28, HCO₃⁻ 12 mEq/L, and C-peptide is undetectable. She is diagnosed with Type 1 diabetes mellitus presenting with diabetic ketoacidosis. After acute management and stabilization, what is the drug of choice for long-term glycemic control?