Define 'strongest'.
Proton pump inhibitors are the clinically accepted standard of treatment for excess gastric acid (omeprazole, lansoprazole and stuff).
H2 antagonists (cimetidine, ranitidine) were the previous standard of care and are OTC in some places that PPI's aren't.
Calcium or magnesium carbonate are also effective for mild stomach acid and for adjusting stomach pH for drugs. (Tums, Rolaids)
Dietary adjustments like avoiding high pH foods or excessive carbohydrate loads can also help.