When comparing modern mortality improvement over the older pre-industrial, pre-modern-medicine regimes, the "most helpful" reductions vary with the age group you're dealing with:
- INFANT (i.e. under 2 years of age) mortality reductions are overwhelmingly due to two things: (1) improvements in reducing childbirth deaths and complications (2) infant vaccinations. Sanitation (but not necessarily clean water) has helped somewhat, but not anywhere near as much as getting the kid out of the mother in good shape, and effective pre- and post-natal care. Vaccines (even though most aren't fully protected until after 2 years of age) have nonetheless stopped cold the huge killers of infants: measles, smallpox, pertussis, etc.
- CHILDHOOD (2-12) mortality reductions are pretty much split between vaccinations and improved clean water/sanitation, maybe with the latter edging out for bigger impact (probably due mostly to reducing malaria, cholera and typhus).
- TEENAGE (13-18) mortality reductions are due to a combination of vaccines (TB, smallpox, polio, and measles being a big here), clean water/sanitation, and trauma medicine.
- ADULT (18-65) reductions are mostly clean water/sanitation, with trauma medicine following up behind. Vaccinations aren't a huge contributor here, since the vast majority of folks died of the major vaccinated diseases before they got to be adults, and thus, a much smaller percentage of people were saved.
- ELDERLY (65+) mortality reductions are heavily improvements in drugs and chronic illness treatments (think cancer and heart disease).
Overall, clean water and sanitation probably win as the single most important advancement in public health, ever, but vaccines are a *very* strong second. Frankly, drugs are at best a distant fourth, behind even improved medical understanding of the human body (enabling more effective trauma and non-drug treatments of common diseases and accidents). Drug improvements really have helped two big categories of people: soldiers at war, and the elderly.