A 62-year-old man presents to a rural emergency department with 45 minutes of central crushing chest pain radiating to his left arm. His blood pressure is 138/82 mmHg, heart rate 92 bpm, oxygen saturation 96% on room air and ECG shows 3 mm ST elevation in leads V2-V4. He has no history of peptic ulcer disease or bleeding disorder and is not taking any antiplatelet therapy. What is the single most important first medication to give, assuming it has not already been administered?