Valve replacement. The aortic and mitral valves are the ones most commonly replaced. The aortic is probably a bit more commonly replaced than the mitral.
Aortic, then mitral. It is close between the two, but the aortic valve is the most commonly replaced, as aortic stenosis is common in older folks. The mitral valve is now often repaired rather than replaced. Pulmonary valve surgery is usally for congenital problems and done in children, while replacement of the tricuspid valve is rare.