What other organization qualifies? There are 2billlion more people in the world directly because of the US. Many have longer legacy's but few have for example removed polio from the world. China and England both played major roles in Smallpox's demise, but you quickly find only major nationstates are in the running.

It's hard to think of world issues on this scale, but we are making a lot of progress as a species. https://en.m.wikipedia.org/wiki/Eradication_of_infectious_di...

PS: US had a major role in the Green Revolution which does have downsides, but as saved more lives than all the 20th century was combined. https://en.m.wikipedia.org/wiki/Green_Revolution the world population was 2.5 billion in 1950.